I wouldn't go the manual way if you are not forced. If you use NetworkManager you can import the configuration either grafically or with nmcli.
It should be sufficiente to modify the network manager service provided by Guix:
(modify-services %desktop-services
[...]
(network-manager-service-type config =>
(network-manager-configuration
(inherit config)
(vpn-plugins (list network-manager-openvpn))))
reconfiguring your system, rebooting and then importing the configuration and set your credentials