We have the ability to create access codes which can be used for unpaid bookings. This can be created under "Set Up" > "Promo Codes" > "Booking Promo Codes":

Select on "Add New Promo Code":

And you may enable the "This promo code applies to unpaid bookings":

The promo code you have set up is fully customizable where you can indicate description, expiration, or if you need to select certain services that this can be applied to.