So you could have "records" list in your datamodel and map your accumlated list variable datapill to it. If you use a message template you would have somehting like: {"upload_request_id":"b3eb858c-e880-4938-bba3-bd5b733dd9b5","records":{{Records}}}A...
Do you have an example body structure that is expected from the API call you will be making via HTTP? This is important to accuracly advise. In many cases you would use a message template for the body itself and would start with the example API bod...
I think you'll need to show the data structure that is received by the triggered event in your start action to get mach advice. Show it with everything expanded if possible.
Is it possible yourt custom attributes are a sub node? If so, it will require iterating thtrogh and checking each "name" / "key" value to find the ones you want to map and seta variable to the value.
