The OnClientEntryAdding client-side event is fired when an entry is about to be added to the entry area.
When the CheckBoxes property of the control is set to either the TriState or the CheckChildNodes , the event will fire only for the node that is being checked.
The event handler receives two parameters:
The instance of the RadDropDownTree client-side object.
An Event argument parameter.
The event can be cancelled - simply call args.set_cancel(true) from the event handler and the entry from dropdown will not be added.