Looking to create a discount code for BOGO on a schedule booking service, not product.

Hello,

my client does psychic readings and wants to sell gift certificates. I've gotten these certificates up and ready for purchasing, but she wants to sell BOGO situation. Buy 3 gift certificates, get one free. I have found how to add a discount on the scheduling portion of squarespace, but it will not give me an option on a BOGO situation. It just automatically adds the discount on the scheduling page without the client having purchased 3 gift certificates. I have found on the ecommerce portion that I can add a discount with a code that allows you to have to spend a certain amount of money to receive the discount, but it's for products and not scheduling/booking portion.

Is there a way around this? 

I also cannot find out how to add the discount code form box to the scheduling checkout page.