The 'well done' comes when you successfully report a convoy. You know the convoy report was sent when you see it at the bottom of your message log (M key) - something like "Large/medium/small convoy, heading ... speed..."
You can send this kind of message only when several merchants of the convoy get reported by the watch crew - the "schiff gesichtet" message. I usually wait for at least 5-6 ships to be sighted and then send the message, which is followed by a "well done" from BdU after a few hours.