First we need to establish the reward!
You know your man best, so you choose what he will be most tempted at achieving!
My suggestion -
- Anal heaven! Your ass available for an intense evening!
or
- Tied up and available! You will tie yourself to the bed, and be his willing subject!
or
- Cum, play and swallow! You get my drift!
Whatever you choose, be elaborate, and tell him what the bonus is IF he makes the challenge!
- remember to make it visual, so give him a tiny "preview" of what he may win!
And remind him of the rules!
If he wins he will get his reward, and a secret bonus! If he looses, he will pay!