You will need to have setup mostly any HTTP server on port 80, which accepts POST request.
This server will be really simple as it needs to respond with single page. The request URL is http://growtopia1.com/growtopia/server_data.php or when alternative URL is requested, then http://growtopia2.com/growtopia/server_data.php.
You might of course notice that you don't own domain growtopia1.com nor growtopia2.com, so you will have to use clever trick. There are few possibilities, but most common is modifying system file called /etc/hosts/ to route all requests from those domains to our own IP address. But you also have variety of others, like using your own DNS server or modyfying growtopia executable.
But now let's look what should be the content of this file:
server|127.0.0.1
port|17091
type|1
#maint|Maintenance message
beta_server|127.0.0.1
beta_port|17091
beta_type|1
meta|localhost
RTENDMARKERBS1001
And now explain all of the stuff one by one.
server - IP adress to which should client connect.
port - Port to which should client connect.
type - I have no idea what it does.
maint - This is maintenance message, if you remove # in front of it, then it will show this text. It is usually used when your ENet server is down.
beta_sever - Same as server, but used when client is in beta mode.
beta_port - Same as port, but used when client is in beta mode.
beta_type - Same as type, but used when client is in beta mode.
meta - This is string which is put into logging packet, no idea what is it's real use.
RTENDMARKERBS1001 - This is marker for end of data (it is from ProtonSDK).
Also there is somewhat important note. You should be using just \n character to separate those lines and not \r\n.