It doesn't look like this feature will be able to help you, the current ui appears to be sufficient to handle your configuration.
Go to Services->VPN->Wireguard, add a "New Wireguard configuration", edit it, check the "Enable" box and fill in the "Private Key" and "IP Adresses" fields, then add a new peer, edit it, set the "Public Key", "Allowed IP", "Endpoint host" and "Endpoint port" accordingly. Don't forget to check the "Route the Allowed Ips" box. Depending on the network, a non null value for the "Persistent Keepalive" may be required.
Then go to Network->Firewall->General Settings, set the input output forwarding rules as required, set the wireguard->lan masquerading checkbox to On.
If all is well, you should be able to ping the "Endpoint Host" at least.
Beware, with AllowedIPs=0.0.0.0/0,::/0 all your traffic will go through the wireguard interface, you may need to restrict it.
Regards,