Much like any other doomsday weapon that LEGION has brought to Seoul, this might actually appear on your world map after you've cleared some Outposts and have gone some distance into the story. If you open your map and find some icons different from the Outposts, Relic stands, or Gremlin hideouts, it might be one of the doomsday weapons. If it isn't the one you're specifically looking for, go clear it and then check your map after to see what has spawned up instead.