If you're first to use the mudfish v4, this page is for you. With the following steps, you can check that it's applied properly or not. I'm assuming that you'd installed the mudfish on your system.
First please run Mudfish Launcher and sign-in with your mudfish account.
Visit 'Item → Buy Items' menu and click 'Site' tab. There you can find 'What's my IP?' item and buy it. (No any credits required because it's free to use)
With the latest release (at least v4.2.2), the configuration would be done automatically so please wait a moment until it shows that it's done.
Visit https://myip.mudfish.net site. If it works correctly, the site would say "It's working."