INVALID_MULTIPART_REQUEST

$response = Http::withHeaders([
    'Authorization' => 'Bearer XYZ',
    'X-API-Key' => 'ABC123',
    'Content-Type' => 'multipart/form-data',
])->post('https://cpf-ue1.adobe.io/ops/:create', $data);

In Laravel i'm trying the endpoint to create a PDF. I use the example data from the document services api documentation: https://documentcloud.adobe.com/document-services/index.html#post-documentGeneration

{
    "cpf:engine": {
    "repo:assetId": "urn:aaid:cpf:Service-52d5db6097ed436ebb96f13a4c7bf8fb"
  },
  "cpf:inputs": {
    "documentIn": {
        "dc:format": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
      "cpf:location": "InputFile0"
    },
    "params": {
        "cpf:inline": {
            "outputFormat": "pdf",
        "jsonDataForMerge": {
                "customerName": "Kane Miller",
          "customerVisits": 100,
          "itemsBought": [
            {
                "name": "Sprays",
              "quantity": 50,
              "amount": 100
            },
            {
                "name": "Chemicals",
              "quantity": 100,
              "amount": 200
            }
          ],
          "totalAmount": 300,
          "previousBalance": 50,
          "lastThreeBillings": [
                    100,
                    200,
                    300
                ],
          "photograph": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mP88h8AAu0B9XNPCQQAAAAASUVORK5CYII="
        }
      }
    }
  },
  "cpf:outputs": {
    "documentOut": {
        "dc:format": "application/pdf",
      "cpf:location": "multipartLabel"
    }
  }
}

I get the following reply:

array:5 [  "requestId" => "fmbhttWQ9VUrmMYuuz2sMjvc28CclxA1"
  "type" => "Bad Request"
  "title" => "Not a multipart request. Aborting."
  "status" => 400
  "report" => "{"error_code":"INVALID_MULTIPART_REQUEST"}"]

What's wrong here?