ConfirmButton Properties:

TargetControlID - The ID of the button or link for this extender to operate on.

Confirm Text - The text to show when you want to confirm the click.

Code:

 <%-- ScriptManager control manages client script for AJAX enabled ASP.NET pages.This enables partial-page rendering and Web-service calls.You have to used this if you want to use ajax control toolkit--%>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
  
<cc1:ConfirmButtonExtender ID="ConfirmButtonExtender1" runat="server" TargetControlID="Button1" ConfirmText="Do you want to submit the page">
</cc1:ConfirmButtonExtender>
  
<asp:Label ID="Label1" runat="server" Text="Enter your email id"></asp:Label>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Submit" />

Run the project:

When you click submit button then confirmation popup will open like