Programmatically adding triggers to an UpdatePanel - Microsoft AJAX Library and ASP.NET 2.0 AJAX Extensions
If you want to add a control to fire the UpdatePanel, you have to add the trigger. You add this through the ScriptManager control. The method is .RegisterAsyncPostBackControl(ControlName);
I put this call in the Page_PreInit() method. I put it there because of some other Atlas stuff was doing that needed an early place in the page lifecycle. .RegisterAsyncPostBackControl() will also work in the Page_Load() event.
Original post: http://morewally.com/cs/blogs/wallym/archive/2006/09/20/359.aspx