Master Pages

http://weblogs.asp.net/caseyroach/archive/2008/07/10/dismiss-the-modalpopupextender-with-esc-or-enter-key.aspx

To get of a server control in master page through JavaScript is
   var id=documnet.getElementById('<%= controlID.ClientID %>');


var id=documnet.getElementById('<%= txtUserId.ClientID %>');