While on your free trial, you have full access to all PRO features, including the challenges and rewards dashboard!
So, while on a free trial, if you want to turn on automated gift card rewards, simply enter your credit card info on the PRO Admin > Account page. (Not to worry, just because we have your credit card, we won't start billing you for the PRO subscription, just the gift cards.) Or, if you simply intend to have non-gift card rewards, on your PRO Admin>Challenges page, you can set your goal and then just choose the "OTHER" rewards type, which allows you to describe your reward.