You do NOT need to book through AMEX Travel. I just applied this to three reservations in the last week.
The agent logs into a special AMEX portal, then types in the customer’s Amex card number to have the system validate that it’s a Platinum card. Then you input the cruise reservation details into a from, like the sail date, ship, res # and total cost and then submit and it generates a confirmation number. Then you call a special number at the cruise line and they ask you for the confirmation code and then they add the OBC (and sometimes other perks like bottle of wine) to the reservation.
That’s it. It’s super easy for agencies to do but they do have to have their agency registered with the special AMEX website.