When you send a booking request, at check-out you can choose add-ons to be added to your future booking.
If you have a confirmed booking and realize that you need to add some add-ons, contact the host through the booking chat and ask them to add add-ons to your confirmed booking.
The host can request a booking update on their end. You will receive a notification in-app and via email and will be asked to confirm the update. Review and approve the updated details.
Once you approve the updated booking, you will be charged for the add-on.