I'm trying to book a BKK-CMB-CDG return ticket in J online but can't find the way to enter the stopover part? how should I do??

Besides I tried to book that ticket from an agency today but they could only book me on the UL metal between CMB and CDG, while I'd prefer flying on monday morning, which means on EK metal via DXB . I've had a CDG-CMB ticket routed that way a few months ago so I'd guess it should still work.
Or it's no longer possible?

Thanks for shedding some light.
Luis