Hi,
first of all - be aware that it is high risk off getting banned.
seconds - im not sure if it is the "perfect way"
edit the bounties.xml only the GameParams need to be changed: isPrivate="False" --> Public game, id="bounties" for tagging your game as a bounties game, letting other players join.
<GameParams act="OpenWorld" isPrivate="False" numGames="-1" id="bounties"/>