HTTP service URL for system external IP detection
Use this field to find your server's public IPs—IPv4 and IPv6—using a web service. If your server uses IPv6, enter two URLs separated by a space: one for IPv4-only and one for IPv6-only. If not, enter just the IPv4 URL.

The default pair is http://v4.software.virtualmin.com/cgi-bin/ip.cgi http://v6.software.virtualmin.com/cgi-bin/ip.cgi. You can also use other services like https://api.ipify.org https://api6.ipify.org.