You should try out like this:
<asp:UpdatePanel runat="server" RenderMode="Inline">
<ContentTemplate>
<asp:Button ID="btnPost" runat="server"
Text="Validate and Post"
OnClick="btnPost_Click"
CssClass="modern socle"
Style="width: 150px; padding: 0px; height: 30px;" />
</ContentTemplate>
</asp:UpdatePanel>
<asp:Panel ID="PleaseWaitMessagePanel" runat="server" CssClass="modalPopup" Height="120px" Width="330px">
Wait a moment...<hr />
While we are sending your email to KS mail server.<br /><br />
<img src="CSS_JS_Img/ContactUsForm/images/mailInOut.png" alt="" style="width: 20px; height: 20px" />
<img src="CSS_JS_Img/ContactUsForm/images/win8_Preloader.gif" alt="Please wait" />
</asp:Panel>
<asp:Button ID="HiddenButton" runat="server" CssClass="hidden" Text="Hidden Button" ToolTip="Necessary for Modal Popup Extender" />
<asp:ModalPopupExtender ID="PleaseWaitPopup" runat="server"
TargetControlID="HiddenButton"
PopupControlID="PleaseWaitMessagePanel"
BehaviorID="PleaseWaitPopup"
BackgroundCssClass="modalBackground">
</asp:ModalPopupExtender>