Merhaba N11 Api Kullanarak Json Post İşlemi Yapıyorum Fakat Sürekli Request part 'data' not found Hatası Almaktayım

Post Ettiğim Kod Aşağıdaki Gibidir

<?php
$data = array( 
		'status' => 'NEW',
		'buyerName' => '',
		'orderNumber' => '',
		'recipient' => '',
		'productSellerCode' => '',
		'period' => array( 'startDate' => '', 'endDate' => '')
);
$data_string = json_encode($data); 
$postfields = array();
$postfields['data'] =$data_string;

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://api.n11.com/rest/secure/order/list.json?appkey=xxx&appsecret=xxx");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_VERBOSE, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postfields);
curl_setopt($ch, CURLOPT_HEADER, true);
curl_setopt($ch , CURL_HTTPHEADER , "Content-Type: multipart/form-data" );
$output = curl_exec($ch);
echo($output) . PHP_EOL;
curl_close($ch);
?>