Yea, the key is just that when you make the booking, whether via BA or AS or some other partner, that you leave the FF# blank (or have the AA# in the reservation from the beginning).

When booking via BA, as has been noted you make the booking for "someone else" but enter your name and other details, then add your AA# later or at the end of the booking flow.

Otherwise with BA if you don't book for "someone else" then your BA# is attached to the reservation from the beginning, and this is what causes issues to arise down the road when you try to change the FF#.