通过API向Greenhouse提交申请

I'm trying to submit an application to Greenhouse with the following way:

$url = "https://api.greenhouse.io/v1/boards/{MY_BOARD_TOKEN}/jobs/{MY_JOB_ID}";

$args = [
            'headers' => [
                'Content-Type' => 'multipart/form-data',
                'Authorization' => 'Basic ' . base64_encode('{MY_API_TOKEN}'),
                'Cache-Control' => 'no-cache',
            ],
            'body' => $form,
        ];

$response = wp_remote_post($url, $args);

But I'm getting the following error:

{"status":400,"error":"Failed to save person"}

My $form looks like this:

[
    'first_name' => 'John',
    'last_name' => 'Doe',
    'email' => 'john@doe.com',
]

I'm sure the credentials are OK.

Thanks in advance,

Status code: 400 Bad Request

The 400 (Bad Request) status code indicates that the server cannot or will not process the request due to something that is perceived to be a client error (e.g., malformed request syntax, invalid request message framing, or deceptive request routing). Link

Means that need to double check you request to API.

I suggest to test it in some other tool and copy&paste request after that.
You could use Restlet Client - REST API Testing for checking.