When I choose NFS for the mount (a Synology drive) it does not let me enter a password / username.
NFS share permissions are handled on the file system (server side). You'll need to add users to the NAS and then change the permissions of the share on the NAS to give your users the desired permissions. The user on Runeaudio is root:root.
You won't have to do anything on the client side of things.
Please check the forum for a solution (for example see here)
Frank