Zero SSL API POST Request Format?

Question

I need zero ssl API post request format

$response = Http::post('https://api.zerossl.com/certificates', [
    'certificate_domains' => [$domain],
    'certificate_csr' =>  $csrContent,
    'certificate_validity_days' => 90,
    'strict_domains' => 1, // Adjust as needed
    'access_key' => $apiKey,
]);

Above code is my post request I got this response:

"success" => false
  "error" => array:3 [▼
    "code" => 116
    "type" => "invalid_json_request_param"
    "info" => "Invalid JSON request parameter."

Help to solve I think I have problem on post request