try this
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>
<div>
<asp:Panel ID="Panel2" runat="server" BorderColor="Red" BorderStyle="Solid" Width="30%"
BackColor="#EEEEEE">
!<span class="style1">!! Have you informed to your Seniors? !!! </span>
<br />
<asp:Button ID="OkButton" runat="server" Text="Yes" />
<asp:Button ID="CancelButton" runat="server" Text="No" OnClick="DoSomething" />
</asp:Panel>
<asp:ModalPopupExtender ID="Panel2_ModalPopupExtender" runat="server" TargetControlID="Button1"
PopupControlID="Panel2" DynamicServicePath="" >
</asp:ModalPopupExtender>
<asp:Button ID="Button1" runat="server" Text="" style="visibility:hidden" />
<asp:Button ID="Button2" runat="server" Text="Click" onclick="Button2_Click" />
</div>
protected void Button2_Click(object sender, EventArgs e)
{
Panel2_ModalPopupExtender.Show();
}