Exactly. And because of that reason I always recommend after booking to check your ticket details to make sure that the booking class is listed. Because afterwards your options are much more limited to obtain that information.

the OP will need some kind of confirmation that shows the booking class. Could be an email when booking, the e-ticket, or the certificate that FB is asking about.

If all fails you could try to challenge FB as it is not stated on their website that these things are needed. But that will be far from guaranteed to work.