Can't find TargetControlID for MODAL POPUP EXTENDER in C# Code behind

Hi,

I can't able to find targetcontrolid from masterpage control in C# codebehind

Please refer the code give below.

       HtmlAnchor lnk_grn = (HtmlAnchor)Master.FindControl("lnk_Receipt");  // Creating object for masterpage control

        ModalPopupExtender1.TargetControlID = "lnk_grn"; // assigning created object as TargetControlID 

        ModalPopupExtender1.Show();

The code I tried for modal popup extender taken from

http://www.aspdotnet-suresh.com/2012/05/aspnet-ajax-confirmbuttonextender.html

Me too trying from my end.

Narendran N