Keycloak[1] is self-hosted and widely used. Kratos[2] is also self-hosted but API only, but on the plus side won't have problem with "josh-api" like GP described above.

[1] https://github.com/keycloak/keycloak

[2] https://github.com/ory/kratos

Is JumpCloud any good? (not a recommendation)

@PostMapping(value="/createAccount")

public String createAccount(String user, String pass){

   var hashmap = readUserMapFromDisk(MYUSERS.PATH_TO_TXT_FILE);

   hashmap.putIfAbsent(user, pass);

   writeToDisk(hashmap, MYUSERS.PATH_TO_TXT_FILE);

   return "All done " + LocalDateTime.now();

}

EDIT: Just imagine the string args are in a more complex object that can be serialized to JSON