Submit PDF form fields (Acrobat) to HTTP Post using Application/JSON

Question

Background:

1. I have created a Microsoft Sharepoint list, with two columns (forename, surname).
2. I have created a Adobe Acrobat PDF form, with two fields (forename, surname) and a submit button.
3. I have used MS Power Automate (aka MS Flow) to create an automation process, that when a HTTP Request is received, to POST fields from the PDF into the SharePoint list (this uses JSON request schema).

Problem:

When I fill out the form in the PDF and click submit, the SharePoint list is populated - however, the fields are blank. So the connection works, but the data is not being carried over.

When I create a HTML form and submit the form, using application/json content type, the data is transferred. See this tutorial that explains how this works: YouTube Tutorial

So... my question is - how to I set the content type as application/json in Adobe Acrobat for PDF forms? Or is there something else I'm doing wrong?

Below is a paste of the JSON I have used in the flow, within 'When a HTTP request is received' (Request Body JSON Schema):

{
    "type": "object",
    "properties": {
        "forename": {
            "type": "string"
        },
        "surname": {
            "type": "string"
        }
    }
}

Thanks