Yup. I merged the feature without documentation
Hopefully somebody helps:
You'll need the url of your nextcloud instance, like "https://nc.example.com" or "https://example.com/nc", your user name within that nc instance and an app password from "NC-> Security -> Devices & sessions"
If that stills fails you'll need to look at the logs and tell us what happens.
There's no reason for it to not work with a subdomain if your nginx/apache proxy is configured properly.
Nextcloud has a tool on its web UI to check about misconfigurations, make sure there's no problem on reverse proxy if you're using Nextcloud AIO kind of solutions.