Some Seabourn specific experience here - I usually put down a FCD while on board but don't actually book any reservations. When I get home, my agent already knows I've put the FCD down (they notify her if she booked the sailing I was just on... although you can also request they *not* do that!). From there, when I'm ready I just call her and have her book my Seabourn cruise.

She also handles all the miscellaneous details for me past my reservation like making sure my referral onboard credit and shareholder onboard credit get applied, adding my Virtuoso amenities, booking private (non-cruise line shore excursions), and she always throws in extra onboard credit too.