To do this, you simply enter the same information about the event you’re betting on, but before you press “Add this bet”, select Lay in the Type drop down and you’re done.
Lay bets, or betting against an outcome, uses another calculation for ROI and of course another way of calculating the result. That’s nothing you need to worry about though, since it still all is done automatically once you settle or update a bet.
This feature will be eventually be available for Pro users only, but it’s open to everyone for a few days to let you try it out.
Good luck with your trades and let us know how it works out for you!