Set permanent route to IPv6 subnet on interface

[Jeremy Ardley <jeremy ardley org>]

What is the best NetworkManager method to add a permanent route to an
ipv6 subnet on an interface?

I can add a temporary route outside of NetworkManager thus:

ip -6 route add 2001:db8:c111:b000::/56 dev eth0

Where should this be done in NetworkManager and what is the correct syntax?

(in this instance the interface eth0 also has an assigned ipv6 /128
address in the subnet range)