Same way as pressing a key to get the password prompt up, but relevant to me is the fingerprint prompt since my reader is the power button and can read if my finger is already on it.
So at the moment the power button is configured to lock, and pressing does that, but pressing again doesn’t wake the screen, though if it could then it’d immediately scan my finger already on the button. Basically one finger lock–unlock.
It’s a inconsequential tweak I wanna try with sddm, but I wouldn’t be surprised if I’d need to slip a patch in to get what I want.