Hi @nixonsam, first of all welcome to the community
You should be able to pass on any JSON structure when enabling the JSON/RAW Parameters option of the HTTP Request node (I assume that’s the one you have in mind):
You can then add any valid JSON object in the Body Parameters field:
{"CartItem": {"GroupId": "Foo", "quantity": 123}}
This would also work for lists of multiple items like so:
[{"CartItem": {"GroupId": "Foo", "quantity": 123}},
{"CartItem": {"GroupId": "Bar", "quantity": 456}}]
Is this what you were looking for? If not, could you share a link to the documentation of the API you are calling?