The functionality that you are trying to accomplish seems like a load on demand functionality of the RadPanelBar. With ASP.NET AJAX RadPanelBar you can try the following: Handle OnClientItemExpand to explicitly trigger the server-side AjaxRequest event as explained in this help topic
. In AjaxRequest, you can find the RadPanelBar instance and the currently expanded item and add content to its child item.
All the best,
the Telerik team
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