Honestly, if you are running HassIO, I would recommend trying out the Nginx Proxy Manager addon. It gives you a pretty simple GUI to use to configure things.
I’m only using that addon, but have no experience with nginx outside of that addon, so I can’t really help guide you for your particular situation, but making a new subdomain is dead simple with the GUI.
Note, you can’t run both at the same time, so stop the main one before starting the manager one.