I watched this video which recommended a tool called Postman for creating and debugging API calls. With it I get more information in the error.
{
"statusCode": 400,
"body": {
"status": "ERROR",
"message": "Unrecognized field: 0"
}
}
The fix was to switch over from raw to x-www-form-urlencoded and put the name-value pairs in one by one. I’m happy to say that this also works in MATLAB now. I still wish I could just load in a structure instead of all of the pairs, but that will be the next challenge.