I’ve literally just lost my MQTT virginity and found it’s far simpler than you’d expect it to be 
In the add-on, just add username and password if you want them…
{
"plain": true,
"ssl": false,
"anonymous": true,
"logins": [
{
"username": "yourusername",
"password": "yourpassword"
}
],
"customize": {
"active": false,
"folder": "mosquitto"
},
"certfile": "fullchain.pem",
"keyfile": "privkey.pem"
}
In config add…
mqtt:
broker: core-mosquitto
username: yourusername
password: yourpassword
…and you’re away.