When you're ready to place your bet, select the BONUS BET button in your bet slip. If you have multiple bonus bets available, select the one you want to use by clicking it. Your stake area should light up in blue. When you're ready, place your bet.
Step by step...
If you have a bonus bet in your account, simply click on your desired market/selections and click the ‘Bonus Bet’ button in your bet slip.
If you have a single bonus, it will automatically select. If you have multiple bonuses available in your account, select the one you want to use by clicking it.
Once you are ready, hit place bet to proceed.