If you play with keyboard and mouse (without controller) - this is what you might want.
- Start/Restart the game.
- Stay in the menu.
- Press Q to switch to Story.
- Press Z to select Options.
- Controls > Controller.
- Lock-On Mode (On-Foot) > Free Aim.
- Lock-On Mode (Horse-Vehicle) > Free Aim.
- Press Escape multiple times.
- Press E to select Online.
Done, you'll be joining free-aim sessions and gain +20 extra exp for each kill.
Note: Settings can't be changed when currently in session (will be grayed out).