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