$("#<%=txtPassword.ClientID %>").keydown(function(event) {

      if (event.keyCode == 13) {

          eval($("#<%=btnLogin.ClientID %>").attr('href'));

     }

  });

http://www.mitchelsellers.com/blogs/2012/06/21/jquery-and-clicking-an-aspnet-linkbutton.aspx

This is a pretty standard jQuery method to listen for the enter key and simply "click" the button.  However, if you are using a LinkButton this code will not work.  The reason for this is that a LinkButton is rendered to the browser as an Anchor tag with a href property that contains a javascript action to trigger the postback.  Click does nothing on the button as there is nothing for it to complete.

jQuery and Clicking an ASP.NET Linkbutton

jquery, aspnet

jQuery