Add your selection to the bet slip by clicking on it. If you have a Bonus Bet available, you'll see "Bonus Bet +" under your selection. Click that to use the bonus instead of your own cash.
Any Bonus Bets you have will then show up for you to choose from. Toggle the one you want to use and hit apply.
Review your bet slip to make sure it's what you wanted, then press "Place Bets".