Just to clarify, are you referring to the Turnkey webmin site?
If so, check this thread [SOLVED] MineOS Turnkey Webmin access on Jessie
It’s a bit dated, but the commands and ports are still the same.
You should be able to log in with any account that has sudo or admin access. The mc account, by default doesn’t have sudo account (I believe), so I created another account for administrative processes.