IP address: The IP address via which the Mosquitto broker is running should be entered here, i.e. that of Home Assistant without the port.
User name: It is best to assign a user name directly for the Nuki in the Mosquitto broker, e.g. mqtt_nuki_smart_lock
Password: Just like the user name, except that the password must not be longer than 32 characters. And I would only recommend letters and numbers, I always had problems with special characters.
Then check the two boxes and everything should work.