!http nested object not being recognized

I need to send a request using the !http method with the following body:

                    "!http": {
                      "method": "POST",
                      "url": "https://app.tatango.com/api/v2/transactional_messages",
                      "body": {
                        "transactional_message": {
                          "number": "${piped_content::0::phone}",
                          "content": "${custom_data::message::value}"
                        }
                      }

However when I test it, the transactional_message object is ignored and only number and content are used.