So, to play on the Underforums Terraria servers on pc, you must first own the game on Steam. After this, right-click the steam shortcut icon, press on properties, and then edit the contents in 'target' and add an extra [-console] so it looks like this.
(make sure you exit Steam from the tab at the top left first)
Then when you reopen Steam, you should see a new tab, it will be called 'Console' and press on that.
Enter 'download_depot 105600 105601 1141292393149860915' into the console, it is slow so be patient. After it is downloaded, it will say the place that the file is downloaded, so you can just go there and open the game.
After that, you are done! All you have to do is just enter the terraria server credentials and voila, you are playing on pc.