Answer
Solution:
The answer can be found here: https://community.shopify.com/c/shopify-design/cart-use-permalinks-to-pre-load-the-cart/td-p/613702 (https://community.shopify.com/c/shopify-design/cart-use-permalinks-to-pre-load-the-cart/td-p/613702)
Note, you don't need any credentials whatsoever to be able to use this feature. You can use it as a customer.
