I go to https://www.jal.co.jp/en/ then click on International flights, click on "Manager my International Booking", click on "Manage booking". Select "SEARCH BY RESERVATION INFORMATION", enter the flight number of the first flight, date of departure, last name, first name and finally the reservation number, which is the PNR from BA (6 digit/alpha code).

That brings up my reservation, which allows to select seats and food.