When booking with OTA, how can I make sure that Frequent Flyer gets attributed to my intended Program?

Sometimes I noticed that many these sites do not provide a place to input any Frequent Flyer at all.
(for example Ovago)

I called a couple of them and half of the time the rep who picked up either say it's not possible, or they don't know.
The other half say to call them after you book it to add it.

I'm a bit worried that even if I call and they add it, it may end up not working because "some people made some mistake".

How do people deal with this?

Thanks!