How do I add a tracking id when I have clicked on buy shipping

I am trying to improve our VTR with Amazon, they have suggested we use ‘buy shipping’. The problem is I can’t add the tracking ID to the order.
Amazon help have suggested I add it once the label has been produced (I can’t! there’s nothing to click on), they have suggested I do it before the label has been produced (you don’t have a tracking reference until the label has been produced).
I have such a long thread on the case log, I’m just getting no clear answers.