Get JWT with custom Claim in the body

Hello, I am trying to perform a test implementation with KeyCloak, what I am looking to do is generate a jwt token with keycloak with a GET or POST request with a CUSTOM variable and KeyCloak return the JWT with that variable in a CUSTOM CLAIM.

Is this action possible?

Or am I required to have users previously added to KeyCloak?

I hope someone can get me out of this doubt.