What you need to do is to define a RoutedEvent
inside of the UserControl and to raise it inside of the DragDropCompleted method. You can do that adding a property inside of the CustomDragDropBehavior which will hold the instance of the UserControl and you will be able to raise the event the following way:
For your convenience I have prepared a sample project which demonstrates the exact approach - when you drag and drop appointment with the control key pressed a MessageBox will appear.
Hope this will help you to achieve the desired.
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