I looked at the fare rules of the ticket and there doesn't seem to be any advance purchase rule, so... you could show up in Bangkok and buy the ticket there and then as long as there are seats in the required fare bucket (Q class) ^
Advanced reservations and ticketing restrictions:
RESERVATIONS ARE REQUIRED FOR ALL SECTORS.
TICKETING MUST BE COMPLETED WITHIN 72 HOURS AFTER
RESERVATIONS ARE MADE.
Cheers