It's really dependent on the client man.

Usually it's just by putting an @ & then start typing their handle.

It is a bit flakey - sometimes i think your client hasn't quite figured out who you care about (your follows) to match to the handle. It also used to be case sensitive in some clients but increasingly less so.

Typing @ & then pasting their xpub is another way.

We're all just muddling our way through this 🤙