There is no configuration setting for setting up a proxy server, within phpList.
Your general approach is to set the fqdn of the phpList machine to the fqdn of the proxy server, and when the click urls/unsubscribe urls arrive at the proxy server, have that machine forward them to phpList machine…
The setting for the fqdn of the phpList machine is “Config”/“Settings”/“Website address (without http://)”
The phpList machine will send responses to these urls, so make sure that’s possible, too.
Reverse proxy (for handling incoming requests) is not the problem. That seems to be working.
The problem is the outgoing requests from phpList.
PhpList is making a number of HTTP/S calls, for example requesting RSS Feeds from phplist.org.
I dont know how to set a proxy for those requests.
The problem is that then I would need to enable full HTTPS internet access from our phplist server. Unless we can restrict by IPs or URLs requested (via proxy whitelisting).