In .aspx code
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="htmlextender.aspx.cs" Inherits="htmlextender" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit.HTMLEditor"
TagPrefix="asp" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:TextBox ID="txt" runat="server" textmode="MultiLine"></asp:TextBox>
<asp:HtmlEditorExtender ID="txt_HtmlEditorExtender" runat="server"
Enabled="True" TargetControlID="txt">
<Toolbar><asp:Undo />
<asp:Redo />
<asp:Bold />
<asp:Italic />
<asp:Underline />
<asp:InsertImage />
</Toolbar>
</asp:HtmlEditorExtender>
<asp:HtmlEditorExtender ID="HtmlEditorExtender1"
TargetControlID="TextBox1" DisplaySourceTab="true"
runat="server">
<Toolbar>
<asp:Undo />
<asp:Redo />
<asp:Bold />
<asp:Italic />
<asp:Underline />
<asp:StrikeThrough />
<asp:Subscript />
<asp:Superscript />
<asp:JustifyLeft />
<asp:JustifyCenter />
<asp:JustifyRight />
<asp:JustifyFull />
<asp:InsertOrderedList />
<asp:InsertUnorderedList />
<asp:CreateLink />
<asp:UnLink />
<asp:RemoveFormat />
<asp:SelectAll />
<asp:UnSelect />
<asp:Delete />
<asp:Cut />
<asp:Copy />
<asp:Paste />
<asp:BackgroundColorSelector />
<asp:ForeColorSelector />
<asp:FontNameSelector />
<asp:FontSizeSelector />
<asp:Indent />
<asp:Outdent />
<asp:InsertHorizontalRule />
<asp:HorizontalSeparator />
<asp:InsertImage />
</Toolbar>
</asp:HtmlEditorExtender>
<br />
<br />
<asp:Label ID="Label1" runat="server" Text=""></asp:Label>
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
</div>
</form>
</body>
</html>