how to show popup on right click

<script type="text/javascript">

    function HideModalPopup()

      {

        var test= $find('ModalPopupExtender1');        

        test.hide();

      }

      function showModalPopup()

    {

        window.parent.document.getElementById('ModalPopupExtender1').show();

    }

    

    </script> 



<asp:ImageButton ID="ShowBtn" runat="server"  />

<Ajax:ModalPopupExtender

                        ID="ModalPopupExtender1" runat="server" CancelControlID="imbOK"

  OkControlID="imbOK" OnCancelScript="HideModalPopup()" PopupControlID="pnlPopup"

                        TargetControlID="pnlPopup">

                    </Ajax:ModalPopupExtender>