It took me a few days, but I managed to get it working. If somebody wants to create this too, and can’t get it to work, send me a message.
Shopify is changing its API to Storefront Cart API - this is very important to know. This is the URL with more information: Migrate your app.
I made use of the HTTP module instead of the Shopify API module. This discussion helped me a lot: Crafting a GraphQL Query - #2 by vendy. The comment of @jtraderj especially, many thanks for this!
This is how he did it:
