Shelly Plus Wall Dimmer and MQTT

I am using HA, Node Red, and Mosquitto in a Docker container. It's been a long time since I setup my other Shelly Plus wall dimmers. I can't get the new one to connect. I have the ip address of the mosquitto server. I have the password for the broker. I'm just not sure of the user name, and I don't know how to confirm those values. Any thoughts?
Thanks for your help.
Steve

The user name in Mosquitto ? It is stored in a password file whose name, and location, is defined in the moquitto.conf file.