Hi, I found out that the endpoint that links products to deals was missing the product_id argument from the Pipedrive API docs. The endpoint itself is working as you would expect.
In technical terms it’s about post request to the endpoint /deals/{id}/products.
The Pipedrive dev team has been contacted and this should be fixed soon. Just wanted to inform others that have been scratching their heads.
You can read further explanations about the product-deal issue from my blog.
Welcome to the community @mikaelahonen 
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Thank you for bringing awareness to this! We’ll update the thread once the product_id parameter has been documented. In the meantime, we also suggest staying tuned to our Changelog as that is where we announce all updates first. 
@Helena_Sul after half a year, this critical but tiny issue hasn’t been fixed on your end. I think every report a frustrated developer sends to the customer using Pipedrive is a bad thing to your business. According your API documentation it is actually not possible to add products to a deal.
Hi @teemur!
Thank you for sharing valid criticism! The reason for the delay has been a human error on my part for which I apologize. We will be adding this parameter to our documentation ASAP and I will be updating this thread!
The underlying functionality of the endpoint is working and the product_id parameter can be added to the POST /deals/{id}/products request as a body parameter to add a product to your deal.
Hi!
Update: The product_id body parameter has been documented for the POST /deals/{id}/products endpoint.