So, I downloaded IPNetShareX and it does
not provide a GUI for configuring port forwarding. It seems that IPNetShareX only assists with certain simple configuation of the natd program that comes with Mac OS X. This is the same program that is enabled by Apple's Internet Sharing technology.
Nevertheless, I think you want to use IPNetShareX instead of Internet Sharing, anyway. The reason is that when natd is run by IPNetShareX it pays attention to /etc/natd.conf, but when it's run by Internet Sharing it doesn't. I think. What this means is that setting up port forwarding "sticks" (persists across reboots) when using IPNetShareX but might not when using Apple's Internet Sharing. I think (again).
Anyway, the makers of IPNetShareX provide
some simple instructions for enabling port forwarding using natd. In theory, these instructions are natd-specific, not IPNetShareX-specific, so they might work with Apple's Internet Sharing, too. Or, they might not. You could try following the instructions first, before switching from Internet Sharing to IPNetShareX, to see if they do the trick. Be sure to test if they work across a reboot.