Example

CURL *hnd = curl_easy_init();

curl_easy_setopt(hnd, CURLOPT_CUSTOMREQUEST, "POST");
curl_easy_setopt(hnd, CURLOPT_URL, "https://app-x.nipamail.com/v1.0/transactional/post?accept_token=NPAPP-IGBmfucf0Np567WzwAH1KuRovnszt9r4HJPJ2U4SRhPk1pCeBXEi7nUkixLby8qwwwymbXwtV3gjOMeodNKlcAChYrZ0D8TQv60208fxXIZlJ7www");

struct curl_slist *headers = NULL;
headers = curl_slist_append(headers, "cache-control: no-cache");
headers = curl_slist_append(headers, "content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW");
curl_easy_setopt(hnd, CURLOPT_HTTPHEADER, headers);

curl_easy_setopt(hnd, 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--");

CURLcode ret = curl_easy_perform(hnd);