A bet? Yea, you're gonna loose. Magick works in subtleties, in feelings, in non-visible ways, if you were going to shoot fire or change a frog into a prince, magick cannot and does not do these things. It's more spiritual, especially Wicca which is less about spell casting and more about spiritual fulfillment.
If you want to cast a spell to win a bet you are already in a bad place as your mind is clouded with desire, but you could cast a luck or wealth spell on yourself or a friend. They happen into a string of good luck you can thank the spell. Not much else you could try though.