To start using RadCallout, just drag it from the toolbox and drop it onto the form. It will be placed in the components tray.
Select RadCallout and click the small arrow on the top right position in order to open the Smart Tag.
It is possible to associate any control available on the form. UserControls are also supported. Just set the Associated Control property from the Smart Tag.
AssociatedControl: allows you to set the associated control.
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RadCallout allows adjusting the arrow direction, close action and other properties at design time: