move the focus from textbox to ajax combobox on enter key

I want to move the focus from the textbox to an ajax combobox on enter key

 <script type="text/javascript">
     $(document).ready(function () {
         $("#txtGroupSname").keydown(checkForEnter);
         function checkForEnter(event) {
 
             if (event.keyCode == 13) {
                 document.getElementById("cmbUnder").focus();
                 return false;
                 //$("#cmbUnder").focus();
             }
         }
     });
 

txtGroupSname is my textbox and cmbUnder is the ajax combobox.

Any ideas or suggestions please.
Thanks,

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