In order to do this side easter egg you will need to first turn on the power on the map.
Related Guide
How To Switch On Power
After you have done this obtain a Shock Charge by either spawning in with one, picking it off the ground or purchasing it from the Crafting Station. Once you have this simply head over to Beacon Island and pick up a fish that's sat in a bucket on a shelf.
Now that you have this, progress the rounds until a T.E.D.D. Task appears. Head on over to it, activate it to see your challenge and then interact with the right hand side of the machine and you will place down the fish. To make this easier you can do this near the end of a round when only a few zombies remain or pop Aether Shroud like I do and this will give you time to get it done.
Then straight after that throw your Shock Grenade onto the fish to electrify the machine as well as surrounding zombies.
Now complete whatever task you are given to get the top Legendary reward and when complete you will see the Fish on the floor along with an electrical panel.
Now head on over to Eioskallen Square and interact with a blank plaque on the wall opposite the Arenal Machine to build the Fishy Fish Bot.
The trap will automatically activate and will act like a Monkey Bomb and attract surrounding zombies. After a short time it will turn nearby zombies that will then help you out. The machine can then be purchased again after a cool down period for 2000 points