When you place your bet, make sure the Pay Back box is highlighted. Toggle between available tokens by swiping right and left. Head to your Rewards section to view your available token balance.
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When they are available, Pay Back tokens will usually be automatically selected for you in your betslip.
If you click on the token and the colour changes to grey (as shown below), this means that you have un-selected the token. To apply the token, make sure it is highlighted as shown above.