Unisender AND promo code file integration required

The job is that a new user appears in the contact list and he needs to substitute unique promo codes from the file.

1. How do users get on the contact list?
The site is made on Tilda. The user pays for the certificate (only 8 options) and the data is included in the contact list "Buy a gift certificate." Users have created 2 additional summa fields - certificate price, giftcard" - promo code, initially empty value.

How does a file with promo codes form?
In manual format created 5-10 promo codes for each version. You need to be able to download/add new promo codes.

3. How does the Unisender and Webhook API work?
www.unisender.com/ru/supp...
www.unisender.com/ru/supp...