Not sure if this helps:
Zammad supports API calls using oauth2.
https://docs.zammad.org/en/latest/api/intro.html#authentication
However, you need to create the token by logging into relevant user account in Zammad (say create an API user account with relevant privileges first?).
There is an authorization code flow also available to create the token - the endpoints are explained under Settings > System > API > Applications.
However, it all depends on your use case what you are trying to do - Kind of SSO between Keycloak and Zammad?