Show the loading message like (Please wait, Item is being launched for all active Employees .....) when click on button

<asp:Button runat="server" ID="btnLaunch" Text="Launch" Width="120px" OnClick="btnLaunch_Click" ValidationGroup="LaunchFields" CssClass="btnwidth" OnClientClick="OpenLoadingAlert();" />

<script type="text/javascript">

    function OpenLoadingAlert() {

        Page_ClientValidate();

        if (Page_IsValid) {

            SP.UI.ModalDialog.showWaitScreenWithNoClose('Loading...', 'Please wait, Item is being launched for all active Employees ..... ', 60, 270);

        }

    }

</script>