Thank you, @Travelingwithstyle! I filled out the form and nothing happened. After a month or so I asked my TA to require OBC and he was told by Azamara that because of 2 booking numbers no OBC!
As I and also TA had never heard about that I asked to insist and finally we got a written receipt that our bookings are qualified for OBC. Azamara said that in the new IT-system you have to have all your following cruises within o e booking number.
It is not mentioned in my account.