If you don’t mind passing your data through on online service, you can use https://putsreq.com
And write 3 lines of code:
var parsedBody = JSON.parse(request.body);
request.body = parsedBody.payload_fields;
request.forwardTo = 'YOURURI';
For investigating responses coming from an HTTP integration (or the putsreq forward), use: https://rbaskets.in/