PopupModal Close Button Click Event

Question

Hi ...
frnds...


i have used popupmodal extender of ajax toolkit but problem is that
i want to fill the dropdown of backside page  on click event of popup close button

ihave used following source code

       <cc1:ModalPopupExtender ID="ModalPopupExtender1" runat ="server" TargetControlID ="button2" PopupControlID ="pnlpopup" CancelControlID ="btn" BackgroundCssClass ="modalBackground">

</cc1:ModalPopupExtender>

<asp:Panel ID="pnlpopup" runat="server" BackColor="White" Height="500px" Width="500px"  DropShadow="True" RepositionMode="RepositionOnWindowScroll style="display:none">
 <iframe id="frameeditexpanse"  height ="500px" width ="100%"   src="../NewFolder1/hudda_master.aspx" frameborder="0">
    </iframe>
   <asp:Button ID="btn" Text ="Close" runat ="server" onclick="btn_Click" />
    </asp:Panel>          

plz help.............................