PUBG: BATTLEGROUNDS

This will be unlocked through natural gameplay, however if you are having difficulty getting kills or want to get this faster, play a game on Miramar, but don't jump out and you will be forced to jump out, as will all the players who have disconnected whilst the plane was flying over. Simply kill them with your fists for easy kills.
Be warned though, players might reconnect during free fall or doing the same thing, so be warned.

I don't think this is doable (or is at least harder) on Erangal or Sanhok as these have water around the edge of the map, so the disconnected players will land in water, so doing it on them will force you to find a gun first, taking time, and the water will scatter them.