Add your own domain
Every site starts on a public link. A custom domain is optional. Open the project, go to Settings, and enter the hostname you control, such as app.example.com.
Create an A record for that hostname to the address shown in the panel. Leave any proxy off until HTTPS is issued. A proxy in front of the record too early can block the certificate.
The domain has to be one you control. The panel checks the hostname and attaches it to that site only. Removing it in Settings takes it off the site.
The original public link stays available, so you can confirm the site is up before DNS finishes.