The secret achievement, titled What the Phonk, is obtained by activating a certain jukebox and winning a match. A previous text guide says that its location is "in the diner" and that the building itself is "near the bridge." I don't know if this has been changed since then or what, but the jukebox inside of the building with a big DINER sign is not the right one.

Instead, you need to go to this location on the map.


When you get close, this is the entrance you are looking for.


Go inside and to the right you will see this jukebox.


It has a red glow to it initially, and getting close to it will bring up an interact prompt. The default key is 'E'. Press it and the jukebox will be activated, making a sound and turning off its light. You will need to extract at the end of the match, and upon doing so the achievement will pop. I did not have to sit and listen to the entire song.

I hope this helps anybody that got frustrated by this like I did.