To implement your scenario you should dynamically add the AJAX settings for ajaxifying the UserControl. You should always do this in a Page event handler, which is executed each time during the page lifecycle (Page_Load, Page_PreRender) so that the control could be ajaxified when the page is initially loaded.
For more information on how this can be done, please take a look at the last section of the RadAjax and WebUserControls
All the best,
the Telerik team
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