Originally Posted by
Some person
The domestic booking system has a few other oddities. You can't select which frequent flyer programme you use and so there's no way to enter AA/AY/BA etc. membership numbers during the booking. JAL asks for your phone number but gives you no option to specify the country code so they might think that you use a +81 phone number even if you don't.
???
I always enter my BA FFP number to JAL online booking system in Japanese.
A trick is booking as a non member, then you have FFP number option button in booking page.
However, once you registered FFP no, you couldn't change it, you have to call.
Yes, a seat selection could be problem. Forutnately, I have both of BA Gold(GGL), and JAL Diamond, (and CX Diamond, curiously, they gave me a card valid through May 2020 after I stopped to credit to CX FFP).