HMAC Advice/Generation/Replication (keyed-hash message authentication code or hash-based message authentication code)

I work on a team of developers and we are currently developing a checkout platform. We currently have most of the steps complete from adding the product to cart using a backdoor (a specific link which is applied to the session that adds the item to cart) method. The program currently only executes the said tasks which is adding the product to cart when specific conditions on the backend of the targeted website are met. The targeted website has recently implemented additional verification which they refer to as "waiting room" or "splash page" which requires the user to obtain a unique cookie prior to having access to select the item and add it to cart that must be verified with the server of the site when checking out. The issue for us is that using our backdoor method, we are not given this unique cookie. Therefore the request is denied without the unique cookie. I have a lot of documentation files and knowledge regarding adidas but when it comes to this portion we are lost and need additional assistance.