All new Plesk Obsidian servers have this feature enabled by default, while on upgraded servers this should be enabled via a command-line interface with the command:
Use either of the following commands:
# plesk bin admin --enable-access-domain "server.example.com"
In this case, if there is a website with the specified name, it will become inaccessible because Plesk panel will be shown.
# plesk bin admin --enable-access-domain ""
In this case, the panel will be accessible on any domain hosted on Plesk and not having website and via IP address on HTTPS.
This feature can be disabled with the following command. After that Plesk access via default port 8443 only is possible:
# plesk bin admin --disable-access-domain