You can get the needed PanelItem (either by index or using the event argument in the server-side event) and using find the control from the header template by using either of the code lines below:
// when used with the ItemClick event of the RadPanelBar
//find panel item by index when used from a different control
You can also find my simplified runnable page for further reference.
All the best,
the Telerik team
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