By using its ip:port

If you have homeserver1 running stuff and that’s connected to the vps through vpn you make sure your homeserver1 service that you want to proxy runs on the homeserver1’s vpn ip.

For docker this is done by specifying that ip when you expose ports or use 0.0.0.0

I assume you can already ping homeserver1 from the vps by using the vpn address of homeserver1