The package does all the work.
There is no “plex” username in DSM 7 - force changed to PlexMediaServer.
Release notes here.
For your situation , if I understand correctly, you only need to have the server self-clim using the “Plex Claim Token”.
- Uninstall w/ KEEP option (default)
- Install again, select “Plex Claim Token” option
- Click “Get Claim Token” link
- Copy
claim-xxxxxxtoken from the newly opened browser window - Return to the installer and paste the token in the box.
- Click NEXT until it starts installing again
- After about 20-30 seconds, it will respond; telling you it’s claimed for your Plex account name.
Now you can open the server.