How do I purge purgeable space in OS Monterey?

Normally you don't and macOS purges space to free it when needed.

So, let's reframe; why do you think you need to purge space? Are you trying to do something that is failing due to space issues? If so, what are you doing and what is the error you get when it fails?

If you just don't like "purgeable" space and want your storage definitively "in use" or "free" - modern OS don't really work that way anymore, and especially not when SSD are in common use.