[CentOS] IPv6 fixed address using ip token

I'm about to publish a fixed IPv6 address and I understand I can use the ip 
token command to lock the host part of the RA-assigned address to a fixed 
value. But I can't see an obvious place to configure this. The logical 
place would be in the ifcfg-<interface> file. Is there someplace else I 
should set this? NetworkManager? It looks like the new systemd-networkd has 
support for this but my interfaces seem to be configured from the old 
initscripts and the new systemd-networkd isn't installed.

<https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/deployment_guide/s2-configuring_ipv6_tokenized_interface_identifiers>