Fill subform with Zapier

Fill subform with Zapier

Hello,
I am using a Subform for a deal, so our marketing team can add several products and custom prices to the deal, which works fine:

But I want to use Zapier to automatically populate that subform as well, but I can't get it to add the details. Usually I use Line Itemizer and it works fine then, but I can't even fill one row:


How do I have to format it?