You shouldn't have any issue when attaching events to the control in the EditAppointmentDialog. All that is needed is to customize the EditAppointmentDialogStyle and add the needed event to the RadComboBox with x:Name="PART_Categories"
, and in the event handler cast the sender
object as RadComboBox in order to get the SelectedItem.
I created and attached a sample project for you to take a look at. Give it a try.
the Telerik team