To play on a private server follow the following steps:
Type the server IP and download click the button "Download server file".
Make sure that the file is named regionInfo.json and doesn't have additional numbers (e.g. regionInfo (1).json) because you downloaded it multiple times.
Press + R on your keyboard and paste the following:
Press Enter or click "OK".
Copy the downloaded file and paste it in the opened folder.
You can now play on your server by opening Among Us and clicking "Online".
To switch back to the original servers, change the region in the bottom right corner.
To play on a private server, fill in the form, then click "Open in Among Us". You can then switch to your server using the Region Selection menu in the bottom right corner.
You may get a confirmation prompt from your browser to confirm that you want to open Among Us, which you should accept.