Thai Airways Royal Orchid Plus - How to book Y or B class?

Not seeing the options on the web site. Anybody know or do I need to call?

Thanks!

You can select 'unrestricted economy' and then check the exact fare class (Y/B/H/M) on the link during the 'buy the ticket' process.

Or use a TA and explain why you want specifically Y or B - presumably for a cheaper mileage upgrade?