Struggling with s3 multipart upload guzzle for Eventbrite

Question

I'm trying to upload to s3 based on the documentation in Eventbrite but I'm getting nowhere. I think it's down to the structure of my request but I've tried multiple different things and nothing is really working and I get a 'Bucket POST must contain a field named 'key' error.

The instructions are here, so it provides you with a presigned POST object, but I can't figure out how to then provide those details in Guzzle https://www.eventbrite.com/developer/v3/resources/uploads/

I would use the s3client but I don't think it's suitable, as I do not have the region name.

So this is the array back from EB, as given in the documentation

$postFields = [
      'key' => $post_args['key'],
      'AWSAccessKeyId' => $post_args['AWSAccessKeyId'],
      'bucket' => $post_args['bucket'],
      'acl' => $post_args['acl'],
      'signature' => $post_args['signature'],
      'policy' => $post_args['policy'],
];

and I've tried various structures:

$args = [
      'query' => $postFields,
      'multipart' => [
        [
          'name' => $instructions_response['body']['file_parameter_name'],
          'Content-type' => 'multipart/form-data',
          'contents' => $image,
        ]
      ],
]

Or

$args = [
      'multipart' => [
        [
          'key' => $post_args['key'],
          'AWSAccessKeyId' => $post_args['AWSAccessKeyId'],
          'bucket' => $post_args['bucket'],
          'acl' => $post_args['acl'],
          'signature' => $post_args['signature'],
          'policy' => $post_args['policy'],
          'name' => $instructions_response['body']['file_parameter_name'],
          'Content-type' => 'multipart/form-data',
          'contents' => $image,
        ]
      ],
];

Or

$args = [
      'key' => $post_args['key'],
      'AWSAccessKeyId' => $post_args['AWSAccessKeyId'],
      'bucket' => $post_args['bucket'],
      'acl' => $post_args['acl'],
      'signature' => $post_args['signature'],
      'policy' => $post_args['policy'],
      'multipart' => [
        [
          'name' => $instructions_response['body']['file_parameter_name'],
          'Content-type' => 'multipart/form-data',
          'contents' => $image,
        ]
      ],
];

(Plus a bunch of more spurious ones that I can't remember anymore)

Anyway, I'm stuck. I just can't think how this POST data is meant to be structured for it to post to S3. I've gone down the rabbit hole with Xdebug and it's still not clear what is even going on when it makes the request tbh

Request called like so

$upload_response = $this->client->request('POST', $upload_url, $args);

UPDATE: It was pointed out to me that whilst 'key' is lowercase in the error message and what comes back from aws, it's capitalised in the documentation. I changed it to 'Key' and now I get 'Conflicting query string parameters: acl, policy'