You have to provide the token yourself, you can see an example of this in the README. Alternatively, you can register a 'token provider' which will allow you to provide the access token on the fly. This feature seems to be undocumented, so if you'd like to provide a documentation fix that would be appreciated.

I’d find it more convenient if the tokens lifespans (expires_in) were exposed to the KeycloakAdminClient class.

For instance, in a client credentials flow, as of v26.3, if the access token lifespan were to be changed in the realm admin, then the setInterval timeout (as in the example) for the access token renewal would have to be manually updated...

EDIT: I made this PR to address this: #41024