It is not supported scenario to add the User Control directly into the RadAjaxManager settings. However, in order to have the AjaxLoadingPanel appearing for the whole user control you could wrap the control in regular asp Panel and add this panel into the RadAjaxManager settings instead.
Other possibility is to use RadAjaxManagerProxy
inside the user control and use it to assign loading panel to updated controls.
I am attaching a simple project, demonstrating how to show loading panel over user control programmatically (more information can be found in "Adding AjaxSettings programmatically
" help article). Also, commented in the user control you may find another approaching with RadAjaxManagerProxy.
Hope this helps.
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