Add your Button or LinkButton
<asp:Button ID="MyButton" Text="Click Here" runat="server" />
Add a Panel to hold your options with a DropDownList
<asp:Panel ID="MyPanel" runat="server">
<asp:DropDownList ID="MyDropDown" runat="server">
<asp:ListItem Value="1" Text="Option 1" />
</asp:DropDownList>
<asp:Button ID="SaveBtn" Text="Save" OnClick="Save_Click" runat="server" />
<asp:Button ID="CancelBtn" Text="Cancel" runat="server" />
</asp:Panel>
Add your ModelPopupExtender
<act:ModalPopupExtender ID="Mpe1" TargetControlID="MyButton"
CancelControlID="CancelBtn" PopupControlID="MyPanel" runat="server" />
Then add your code behind to the SaveBtn Button
public void SaveBtn_Click(object sender, EventArgs e) {
string selectedOption = MyDropDown.SelectedValue;
}