Hi @brunoamaral the challenge is more that we don't know what the service API you are calling expects in terms of how you have to format the request.
As a start, I would remove the msg.headers settings and let the node encode it how it thinks it should be set.
The format of the payload is slightly unusual - you are posting a form with one property called JSONString whose value is the json encoding of the object with the url and format properties. Is that really what your service expects? Or should the body of the http request have the url and format properties at the top level? I would try setting msg.payload = data and seeing what happens (again, without the msg.headers property being set so the node sets the proper default header values).