Seems like that’s the place to start (Apache is your web server, and we don’t do anything that would make it behave differently for various app servers, proxying is the same in a Virtualmin system as in any other Apache system). Their example looks pretty reasonable:
<VirtualHost *:*>
RequestHeader set "X-Forwarded-Proto" expr=%{REQUEST_SCHEME}s
</VirtualHost>
<VirtualHost *:80>
ProxyPreserveHost On
ProxyPass / http://127.0.0.1:5000/
ProxyPassReverse / http://127.0.0.1:5000/
ServerName www.example.com
ServerAlias *.example.com
ErrorLog ${APACHE_LOG_DIR}/helloapp-error.log
CustomLog ${APACHE_LOG_DIR}/helloapp-access.log common
</VirtualHost>
You’d want to put that into an actual Virtualmin managed VirtualHost. The only bits you need would be the Proxy* rules. The RequestHeader set thing is new to me, though, you may want to read up on that a bit. If it does what I think it does, I probably want to read up on it, too, as that seems convenient for proxying to app servers.