DSM 7 has the “Plex Claim Token” installation method.
This method claims the server for you (you give it a claim token)
during the installation.
If the Syno can communicate with Plex.tv then it’s attached to your account as soon as installation is complete.
you only need open it by http://lan.ip.of.syno:32400/web the firs time to complete initial setup.
This How-To shows reclaiming an existing server but works either way.