Hi Carol Watson,
Currently, adding and removing elements at design-time is not supported. However, you can customize a screentip programmatically. This approach is demonstrated in our Examples application, section GridView >> ScreenTips
. In order to subscribe to the ScreenTipNeeded of the RadGridView instance that is hosted in the dropdown of RadMultiColumnComboBox, you can use the EditorControl
If you have additional questions, feel free to contact me.
the Telerik team
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