Hey,
u do this using ajax
try following code
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:Button ID="btnOpen" runat="server" EnableTheming="False" Text="Open" />
<cc1:ModalPopupExtender ID="btnOpen_ModalPopupExtender" runat="server" DynamicServicePath=""
PopupControlID="divPopUp" TargetControlID="btnOpen" CancelControlID="BtnClose"
Enabled="True" >
</cc1:ModalPopupExtender>
<div id="divPopUp" style="height:250px;width:400px; background-color:Aqua;">
<asp:Button ID="BtnClose" runat="server" EnableTheming="False" Text="Close" />
This is Popup Window
</div>
Best luck