I don't know if this is dependant on you having finished the game or not... (EDIT: It appears that the Peacekeeper Beacon is automatically added to your Gadget menu after liberating 'some' of the Prisoner Hardpoints (i've seen reports that it only takes 1 liberated Prisoner Hardpoint, others say all 12? I had all 12 liberated when this update came out, and this gadget was immediately selectable).
If you go to any Supply Point, open up the Loadout menu, and then go into the Gadget menu, you should just be able to select the Peacekeeper Beacon from the bottom row on the left. After exiting the menu, you can activate the beacon by hitting the
button on your controller.
The game seems a little picky about where you can deploy the beacon as i kept getting the message 'Cannot perform action' every time i tried to deploy it right next to the Supply Point. Just keep moving about and spamming the beacon, at some point it will activate for you. What worked for me was to go somewhere on ground level, near water/the shore and try deploying it there.