Example

$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => "https://app-x.nipamail.com/v1.0/transactional/post?accept_token=NPAPP-IGBmfucf0Np567WzwAH1KuRovnszt9r4HJPJ2U4SRhPk1pCeBXEi7nUkixLby8qwwwymbXwtV3gjOMeodNKlcAChYrZ0D8TQv60208fxXIZlJ7www",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"from_name\"\r\n\r\nThaksinai Kondee\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"from_email\"\r\n\r\nthaksinai@hotmail.com\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"to\"\r\n\r\nthaksinai@ispio.com\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"subject\"\r\n\r\nร่วมฉลอง ครบรอบการก่อตั้ง บริษัท Nipa technology วันนี้ เวลา 18:00 น.\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"message\"\r\n\r\ncontent1\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"reply_email\"\r\n\r\nse55660159@gmail.com\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW\r\nContent-Disposition: form-data; name=\"reply_name\"\r\n\r\nThakweb.com\r\n------WebKitFormBoundary7MA4YWxkTrZu0gW--",
  CURLOPT_HTTPHEADER => array(
    "cache-control: no-cache",
    "content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW"
  ),
));

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}