On the Events tab:
- Press the "New Event" button
- Select "Create a new event from scratch"
- Enter a description
- Select the trigger "A random player is chosen for a prize"
- For "Conditions", enter:
isBountyPrize
- Add Actions
If you have any other events triggered off of "A player busts out" or "A player rebuys", move this event above those or else one of those will likely get triggered instead of this one (since the random prize will be awarded when a player busts out).