Yet, for me, it repeatedly reverted the dropdown menu to Thai's ROP and rejected my TK number each time I submitted the passenger details page. Maybe it doesn't like two passengers on the reservation.

Anyway, later in the day I tried again without bothering to enter any FF details and I was successfully taken to the payment page and I have my reservations for a week in Bali in the new year. I'll go visit the Thai ticketing office in Bangkok at some point in December and get them to add our TK FF numbers manually as indeed you cannot add or change them after the reservation is made because the fields are grayed out. Bravo!