Use FQDN for VPN configuration

Example domains and subs used to protect the innocent -

I own mydomain.com . I would like to use something like vpn.mydomain.com for our SSL VPN connections and not use our public IP address. On my host I’ve pointed vpn.mydomain.com to my public IP address.

I understand that the “Override Hostname” is what I’m looking to use to push out the correct VPN config, however that field insists “You must enter a network IP address”.

How can I accomplish what I’m trying to do?