when target control ID event is not fired in Ajax Modal Popup
so put another control ID for Target Control ID
just for this purpose we are using lblSubmit Control
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="pnlDisplay"
TargetControlID="lblSubmit" BehaviorID="btnSubmit">
</asp:ModalPopupExtender>