You no longer actually book future cruises on the ship. You can book Placeholders for $250 on the Navigator App. The placeholder gives you a 10% discount on a future cruise and 10% deposit on 7 nights or longer. The placeholder is good for 2 years and you must sail before the 2 years is up. If you don't book a cruise with your placeholder your money will be returned to the original form of payment.
However, as of right now, you have to call
DCL to book your cruise with the placeholder. You cannot book a cruise with your placeholder on line.
I believe you can assign a travel agent to your booking.
Otherwise, I book online opening day if there is an itinerary that suits us. I don't use a travel agent or go through Costco because I like to have ultimate control of my booking.