It was an SSL connection with a custom cert chain. So I could easily wrap the HTTP side of things with a SOCKS forwarder which then SOCKS proxied into the VPN and then back out to the other site.

There might have been another way to do it, but given that wrapping HTTP in a SOCKS proxy is easy and ssh has a built in SOCKS proxy, this was quick win.

ps. It's SOCKS and not Socks - for the subject line of this story.

Just a note: if you're forwarding for something that can use socks5, ssh can do that (can be used to share an outgoing VPN from a server you can ssh into, for example - by pointing your browser to your local ssh socks5 proxy. Or for testing a rest/soap/graphql service that's not exposed to the internet).

https://man.openbsd.org/ssh#D

the thing is, port forwarding works for a specific port, whereas socksproxy is a general tunnel.

if you point firefox to use a remote ssh host as socks proxy then you can browse internet as if you were in the ssh host subnet.

it's basically a poor man's vpn, but if all you do is www browsing it works perfectly.