Fix Wordpress API Issue With Sending Request via PHP Post

I need a PHP developer to fix a problem with our API. We believe that this is maximum 2 hours work for an experienced developer.

We are using the plugins Gravity Forms, Gravity Flow and Gravity PDF to send API requests to a third party endpoint https://metpark.zatappeal.com. We need help to check the code for the API and make GravityFlow send files via the webhook to a third party.

The developer needs knowledge of sending multi-part form data requests with JSON and a PDF file.
We are flexible on how the implementation is done, however, the example code that GravityFlow have recommend is here: https://docs.gravityflow.io/article/95-gravityflowwebhookargs

See Example 7 - Post multipart/form-data with single file upload.

Your objective:
1. Update the code if required
2. Test with sending sample requests
3. Test if the requests are being sent successfully (via logs and endpoint responses)

The endpoint we are trying to send data to is https://metpark.zatappeal.com
The payload has been prepared, but help is required to triage why the PDF is not being sent and to make possible code changes.

UK developer preferred for this project.