The only way to keep a control from loosing its focus is to cancel its Validating event. To handle your scenario, please move your logic to the validating event, cancel it when needed and show the popup:
sender, CancelEventArgs e)
Please note that it is important that you disable the control's animations in order to be able to show the popup correctly, since they cause a delay in closing the popup which might lead to undesired behavior:
I hope this helps.
the Telerik team