Each participant must register under their own account so they can access their entry and accept the waiver. You can't register them under your account and pass the entry over later.
Print a QR code linking to the registration page so people can sign up on their own phone.
Offer a unique onsite discount code as an incentive.
Have a tablet available for people without a phone.
Participants can pay by card, or with Apple Pay or Google Pay, on the spot.