How we implement the ConfirmButtonExtender 

Step1: we use the following control on aspx page 

Example:

 <form id="form1" runat="server">
 <div>
 <%--Using ScriptManager or ToolkitScriptManager according toolkit version--%>
 <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
 <%-- using ConfirmButtonExtender here--%>
 <cc1:ConfirmButtonExtender ID="ConfirmButtonExtender1" runat="server"   
 ConfirmText="are you sure submit the page?" TargetControlID="Button1">
 </cc1:ConfirmButtonExtender>
 <%--Add button for submit the page--%>
 <asp:Button ID="Button1" runat="server" Text="Submit Page" />
 </div>
</form>

  Step3: Run the project

Click on submit button