[keycloak-user] User registration outside of Keycloak login form

[Ilya Korol]

You should use your custom Authenticator (see docs for Server 
development -> Authentication SPI). So the idea is to put this 
authenticator (Optional requirement)  inside browser flow right after 
Cookie. It will check whether user is authenticated or not (if user was 
authenticated it means that Cookie worked). Then it will show form that 
you described via its challenge() method, and process user reaction in 
action() method. If you don't fully understand what i've wrote check 
docs and you definitely should get the idea. Cheers.