As of last year, the PG did not have a cruise consultant on board the ship so you could not make an on board booking while actually on board!
Make sure you get the letter that allows you to book using the onboard discount when you get home.
You have either a week or 10 days to make the reservation, deposit and book when you get home. There is a reference number on the letter.
Better yet, email Ngaire from the ship and you can have a REAL OnBoard Booking!