Camper plates are assigned automatically and can change before pick-up for logistical reasons, so a specific plate can't be chosen or reserved in advance.
For ferry or campsite bookings, use the vehicle's dimensions rather than the plate — that's what these providers check when deciding whether they can accommodate you. Most providers also let you add or update the plate closer to your travel date, so you can confirm it once your vehicle is final. Check your provider's policy for their update deadline.
If it helps, customer service can tell you the plate currently assigned to your booking — just note it may still change, and the plate you receive at pick-up is the confirmed one.