That is not where the claim token is used.
Is the NAS running DSM 6 or DSM 7?
Is Plex Media Server installed via Synology Package Center or are you running it in Docker?
Option 1: Web Browser
Point a private/incognito window at the IP address of the NAS. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web.
See if you can login, reach server settings, and claim the server. You should see your server name listed below the Plex Web section, and General under the server name.
Option 2: DSM 7, Uninstall/Reinstall
If the NAS is running DSM 7 and PMS is installed via Package Center (not Docker), then use FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out).
The process is straightforward. You uninstall Plex, then re-install using the Plex Claim Token. This is only for authentication/claiming. Your media files, Plex settings, etc. are not touched.
Option 3: User Credential Reset Utility
If running PMS in Docker, use the User Credential Reset utility. It also works for PMS installations using Package Center.
Download the script to the NAS, login via SSH and run it as root (see the readme on GitHub for details). You will be prompted for a Plex claim token.
