Simple way to use confirm button using ajax controls
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="confirmbutton.aspx.cs" Inherits="confirmbutton" %>
<%@ 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:Button ID="Button1" runat="server" Text="Button" />
<asp:ConfirmButtonExtender ID="ConfirmButtonExtender1" runat="server" TargetControlID="Button1"
ConfirmText="Are you sure you want to click this?"
OnClientCancel="CancelClick">
</asp:ConfirmButtonExtender>
</div>
</form>
</body>
</html>