<asp:TabContainer ID="TabContainer1" runat="server">
<asp:TabPanel ID="TabPanel1" runat="server">
<HeaderTemplate>test tab</HeaderTemplate>
<ContentTemplate>
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
TargetControlID="Button1"
PopupControlID="Panel1"
CancelControlID="CancelButton"
BackgroundCssClass="testcss"
Enabled="True" />
<asp:Button ID="Button1" runat="server" Text="btnpop" />
</ContentTemplate>
</asp:TabPanel>
</asp:TabContainer>
<asp:Panel ID="Panel1" runat="server">
<h1>Popup Panel</h1>
<asp:Button ID="CancelButton" runat="server" Text="Cancel" />
</asp:Panel>