Set Focus on Textbox in modalPopup Panel?

<asp:ModalPopupExtender ID="ModalPopupExtenderRent" runat="server"   BehaviorID="bePopup "  RepositionMode="None"

        TargetControlID="btnshowRent" PopupControlID="divRent" Drag="true"

        PopupDragHandleControlID="divRent">

    </asp:ModalPopupExtender>

<script type="text/javascript">

    function OnPopupShow()

   {

    document.getElementById('<%= txtprovNoteSubject.ClientID %>').focus();

   }

    function SpecialProvAdd()

   {

    try

     {

    

         document.getElementById("<%=hdnGridName.ClientID %>").value = GridName

         var modalPopup = $find("bePopup"); 

                            //this is the BehaviorID from the ModalPopupExtender

          if (modalPopup != null) {

             modalPopup.add_shown(OnPopupShow);

         } 

    }

 catch(er)

 {

}

}

</script>

OnClientClick="SpecialProvAdd();$find('bePopup').show();return false;"