What helped me to solve this problem ist this reply:
https://ecommerce.shopify.com/c/ecommerce-design/t/landing_site_ref-using-it-to-fire-the-appropriate-tracking-pixel-for-shareasale-and-commission-junction-407050#comment-526894
I just want to add that is is really bad style for shopify to declare in their own (!) docu, which is today online, something works, which clearly doesn’t and does not for a while.
It is also not promising, that their own support does neither know this docu nor that it is outdated.
That made us waste two weeks in hope of getting a qualified answer. If we would have not found this solution it would have been reason for us to not use shopify at all.
So yeay to this community and boo to the shopify documentation and support.