The sample demonstrates how to add bookmark into Word for Silverlight via Spire.Doc.

<Application xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
             x:Class="Bookmark_Doc.App">
             
    <Application.Resources>
        
    </Application.Resources>
</Application>

using System;
using System.Windows;

namespace Bookmark_Doc
{
    public partial class App : Application
    {

        public App()
        {
            this.Startup += this.Application_Startup;
            this.Exit += this.Application_Exit;
            this.UnhandledException += this.Application_UnhandledException;

            InitializeComponent();
        }

        private void Application_Startup(object sender, StartupEventArgs e)
        {
            this.RootVisual = new MainPage();
        }

        private void Application_Exit(object sender, EventArgs e)
        {

        }

        private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
        {
            // If the app is running outside of the debugger then report the exception using
            // the browser's exception mechanism. On IE this will display it a yellow alert 
            // icon in the status bar and Firefox will display a script error.
            if (!System.Diagnostics.Debugger.IsAttached)
            {

                // NOTE: This will allow the application to continue running after an exception has been thrown
                // but not handled. 
                // For production applications this error handling should be replaced with something that will 
                // report the error to the website and stop the application.
                e.Handled = true;
                Deployment.Current.Dispatcher.BeginInvoke(delegate { ReportErrorToDOM(e); });
            }
        }

        private void ReportErrorToDOM(ApplicationUnhandledExceptionEventArgs e)
        {
            try
            {
                string errorMsg = e.ExceptionObject.Message + e.ExceptionObject.StackTrace;
                errorMsg = errorMsg.Replace('"', '\'').Replace("\r\n", @"\n");

                System.Windows.Browser.HtmlPage.Window.Eval("throw new Error(\"Unhandled Error in Silverlight Application " + errorMsg + "\");");
            }
            catch (Exception)
            {
            }
        }
    }
}

Imports System.Windows

Namespace Bookmark_Doc
	Partial Public Class App
		Inherits Application

		Public Sub New()
			AddHandler Me.Startup, AddressOf Application_Startup
			AddHandler Me.Exit, AddressOf Application_Exit
			AddHandler Me.UnhandledException, AddressOf Application_UnhandledException

			InitializeComponent()
		End Sub

		Private Sub Application_Startup(ByVal sender As Object, ByVal e As StartupEventArgs)
			Me.RootVisual = New MainPage()
		End Sub

		Private Sub Application_Exit(ByVal sender As Object, ByVal e As EventArgs)

		End Sub

		Private Sub Application_UnhandledException(ByVal sender As Object, ByVal e As ApplicationUnhandledExceptionEventArgs)
			' If the app is running outside of the debugger then report the exception using
			' the browser's exception mechanism. On IE this will display it a yellow alert 
			' icon in the status bar and Firefox will display a script error.
			If Not Debugger.IsAttached Then

				' NOTE: This will allow the application to continue running after an exception has been thrown
				' but not handled. 
				' For production applications this error handling should be replaced with something that will 
				' report the error to the website and stop the application.
				e.Handled = True
				Deployment.Current.Dispatcher.BeginInvoke(Sub() ReportErrorToDOM(e))
			End If
		End Sub

		Private Sub ReportErrorToDOM(ByVal e As ApplicationUnhandledExceptionEventArgs)
			Try
				Dim errorMsg As String = e.ExceptionObject.Message + e.ExceptionObject.StackTrace
				errorMsg = errorMsg.Replace(""""c, "'"c).Replace(vbCrLf, vbLf)

				System.Windows.Browser.HtmlPage.Window.Eval("throw new Error(""Unhandled Error in Silverlight Application " & errorMsg & """);")
			Catch e1 As Exception
			End Try
		End Sub
	End Class
End Namespace

<UserControl x:Class="Bookmark_Doc.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <Button Content="RUN" Height="23" HorizontalAlignment="Left" Margin="286,265,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
    </Grid>
</UserControl>    

using System.Windows;
using System.Windows.Controls;
using System.IO;

using Spire.Doc;
using Spire.Doc.Documents;

namespace Bookmark_Doc
{
    public partial class MainPage : UserControl
    {
        private SaveFileDialog saveFile = null;
        public MainPage()
        {
            InitializeComponent();
            this.saveFile = new SaveFileDialog();
            this.saveFile.Filter = "Word Document(*.doc)|*.doc";
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            //create a word document
            Document document = new Document();

            //add one section
            Section section = document.AddSection();

            AddBookmark(section);

            //save word document
            bool? result = this.saveFile.ShowDialog();
            if (result.HasValue && result.Value)
            {
                using (Stream stream = this.saveFile.OpenFile())
                {
                    document.SaveToStream(stream, FileFormat.Docx);
                }
            }


        }

        private void AddBookmark(Section section)
        {
            Paragraph paragraph = section.Paragraphs.Count > 0 ? section.Paragraphs[0] : section.AddParagraph();

            paragraph.AppendText("This is a sample demonstrates how to use bookmark in Spire.Doc");
            paragraph.ApplyStyle(BuiltinStyle.Heading2);

            //writing simple bookmarks
            paragraph = section.AddParagraph();
            paragraph.AppendText("Simple Bookmark1");
            paragraph.ApplyStyle(BuiltinStyle.Heading4);
            paragraph = section.AddParagraph();
            paragraph.AppendBookmarkStart("Simple_Bookmark1");
            paragraph.AppendText(" This is the first simple bookmark");
            paragraph.AppendBookmarkEnd("Simple_Bookmark1");
            paragraph = section.AddParagraph();
            paragraph.AppendText("Simple Bookmark2");
            paragraph.ApplyStyle(BuiltinStyle.Heading4);
            paragraph = section.AddParagraph();
            paragraph.AppendBookmarkStart("Simple_Bookmark2");
            paragraph.AppendText(" This is the second simple bookmark\n");
            paragraph.AppendBookmarkEnd("Simple_Bookmark2");

            //writing nested bookmarks
            paragraph = section.AddParagraph();
            paragraph.AppendText("Nested bookmarks");
            paragraph.ApplyStyle(BuiltinStyle.Heading4);
            paragraph = section.AddParagraph();
            paragraph.AppendBookmarkStart("Root");
            paragraph.AppendText(" Root data ");
            paragraph.AppendBookmarkStart("NestedLevel1");
            paragraph.AppendText(" Nested Level1 ");
            paragraph.AppendBookmarkStart("NestedLevel2");
            paragraph.AppendText(" Nested Level2 ");
            paragraph.AppendBookmarkEnd("NestedLevel2");
            paragraph.AppendText(" Data Level1 ");
            paragraph.AppendBookmarkEnd("NestedLevel1");
            paragraph.AppendText(" Data Root ");
            paragraph.AppendBookmarkEnd("Root");
        }
    }
}

Imports System.Windows
Imports System.Windows.Controls
Imports System.IO

Imports Spire.Doc
Imports Spire.Doc.Documents

Namespace Bookmark_Doc
	Partial Public Class MainPage
		Inherits UserControl
		Private saveFile As SaveFileDialog = Nothing
		Public Sub New()
			InitializeComponent()
			Me.saveFile = New SaveFileDialog()
			Me.saveFile.Filter = "Word Document(*.doc)|*.doc"
		End Sub

		Private Sub button1_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
			'create a word document
			Dim document As New Document()

			'add one section
			Dim section As Section = document.AddSection()

			AddBookmark(section)

			'save word document
			Dim result? As Boolean = Me.saveFile.ShowDialog()
			If result.HasValue AndAlso result.Value Then
				Using stream As Stream = Me.saveFile.OpenFile()
					document.SaveToStream(stream, FileFormat.Docx)
				End Using
			End If


		End Sub

		Private Sub AddBookmark(ByVal section As Section)
			Dim paragraph As Paragraph = If(section.Paragraphs.Count > 0, section.Paragraphs(0), section.AddParagraph())

			paragraph.AppendText("This is a sample demonstrates how to use bookmark in Spire.Doc")
			paragraph.ApplyStyle(BuiltinStyle.Heading2)

			'writing simple bookmarks
			paragraph = section.AddParagraph()
			paragraph.AppendText("Simple Bookmark1")
			paragraph.ApplyStyle(BuiltinStyle.Heading4)
			paragraph = section.AddParagraph()
			paragraph.AppendBookmarkStart("Simple_Bookmark1")
			paragraph.AppendText(" This is the first simple bookmark")
			paragraph.AppendBookmarkEnd("Simple_Bookmark1")
			paragraph = section.AddParagraph()
			paragraph.AppendText("Simple Bookmark2")
			paragraph.ApplyStyle(BuiltinStyle.Heading4)
			paragraph = section.AddParagraph()
			paragraph.AppendBookmarkStart("Simple_Bookmark2")
			paragraph.AppendText(" This is the second simple bookmark" & vbLf)
			paragraph.AppendBookmarkEnd("Simple_Bookmark2")

			'writing nested bookmarks
			paragraph = section.AddParagraph()
			paragraph.AppendText("Nested bookmarks")
			paragraph.ApplyStyle(BuiltinStyle.Heading4)
			paragraph = section.AddParagraph()
			paragraph.AppendBookmarkStart("Root")
			paragraph.AppendText(" Root data ")
			paragraph.AppendBookmarkStart("NestedLevel1")
			paragraph.AppendText(" Nested Level1 ")
			paragraph.AppendBookmarkStart("NestedLevel2")
			paragraph.AppendText(" Nested Level2 ")
			paragraph.AppendBookmarkEnd("NestedLevel2")
			paragraph.AppendText(" Data Level1 ")
			paragraph.AppendBookmarkEnd("NestedLevel1")
			paragraph.AppendText(" Data Root ")
			paragraph.AppendBookmarkEnd("Root")
		End Sub
	End Class
End Namespace