To activate the beacon in Barotrauma, you must first guide the submarine to the beacon’s location. The location of beacons will usually be abandoned stations with one entrance, which tends to be a door at the top of the station.
Getting to the top of the station is easy but not the only way to get in. You can also use a plasma cutter to cut through the wall. But if you use this method, make sure to bring along a welding tool to seal the hole up.
However, before you even think about heading into the station, make sure you bring one fuel cell as you will need it to get the reactor back online. That said, once you get into the station, seal up any holes or leaks that are flooding the station.
Related: How to ban and kick players from your lobby in Barotrauma?
Once everything is locked up, locate the reactor, which will look just like the one on your ship. Sometimes, you need to repair it, and to repair it will require a wrench and a screwdriver to rewire it. In other cases, you will be able to just chuck the fuel cell in and turn it on with no issues.
After placing the fuel cell in, you will need to locate the control panel and when the power comes back online. Accessing the control panel will allow you to turn the beacon on, which will complete the side objective and activate the beacon.
For more Barotrauma content, PGG has you covered!