Please be aware that there is a difference between openHABian and openHAB.
Current version of openHABian is 1.9 (Releases · openhab/openhabian · GitHub).
If setting up mosquitto via openhabian-config, There will be some credentials per default. Default user is openhabian and password is asked during installation. If you don’t remember which password you chose, you can create new credentials by reinstall (in fact this will only add a new line or change the existing line in the passwd file - located in /etc/mosquitto/). The password itself is hashed, so no luck in looking up in the passwd file.