[keycloak-user] restricting user registration?

Is it possible to configure user self-registration to a new realm such that only users with specific token or link can register? We want to allow self-registration, but do not want it to be open-ended, but rather by invitation where the invitation would be an email or some OOB mechanism that provides the user with a unique 1-time-only code to use to authorize their registration. Is this possible? Thanks, Wyllys Ingersoll