Originally Posted by
Tokyoite
Go to manage booking and access your booking.
*Better to select the JAL website location to be any location outside Japan (Tokyo).
Scroll down to Passenger & contact information and click on "Register of passport information"
Complete your details, including passport number and Traveler Redress Number (if you have one).
Then click save
Tokyoite -- thank you for the above.

As such, I logged in via the JAL
UK site and via the
Manage My Booking tab, but in the
Passenger & Contact Information area there isn't any "
Register of Passport Information" option, unfortunately. (Perhaps, with regard to other geo versions of the JAL website, this option is available, but no such luck via the UK version, sadly.)
Thank you in any case, though, for the information.
All the best.