Salesforce HTTP Callout to Dot Digital

I have a record triggered Lead flow in Salesforce Flow Builder with an async path. In this path I've created an HTTP callout to add a contact to an address book in Dot Digital. The JSON sample request contains an array called dataFields that are all type string. However, Dot Digital seems to parse the strings and return them as one of a variety of data types (boolean, date, integer) in the JSON response received, which causes a 200 message (and the contact is added to the address book successfully) but also an error message saying the data values don't match, which prevents saving.

I need someone to help me pass Lead data to this dataFields array in the Flow Builder (ideally).

API reference is here: https://developer.dotdigital.com/reference/add-contact-to-address-book

A developer sandbox is setup with live link to a (currently unused) address book in Dot Digital.