If you wanna use Content-type: application/json and raw data, seem your data
should be in json format
$ch = curl_init();
$headers = [
'x-api-key: XXXXXX',
'Content-Type: text/plain'
];
$postData = [
'data1' => 'value1',
'data2' => 'value2'
];
curl_setopt($ch, CURLOPT_URL,"XXXXXX");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($postData));
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
$result = curl_exec ($ch);
$statusCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);