Hi Malcolm,
Thanks for your interest in syncfusion products.
You may refer the following online sample link to know more information on displaying WaitingPopup control during the button click.
http://asp.syncfusion.com/sfaspnetsamplebrowser/9.1.0.20/Web/Grid.Grouping.Web/samples/4.0/PopulateData/MasterDetail/cs/MasterDetail.aspx?args=1You may also refer the following documentation link to know more information regarding the working principle of WaitingPopup,
http://help.syncfusion.com/ug_84/User%20Interface/ASP.NET/Tools/Documents/throughdesigner20.htmKindly refer the above links and let us know if this helps.
Regards,
Varalakshmi.R.S