libcurl HTTP POST doesn't work - c++

Question

I'm trying to send HTTPS POST request using libcurl.
Here's the code:
CURL* curl = curl_easy_init();
if (!curl) return "";
CURLcode res;
std::string request = "https://some-site.com/test.php";
std::string response;
curl_easy_setopt(curl, CURLOPT_URL, request);
curl_easy_setopt(curl, CURLOPT_POST, true);
curl_easy_setopt(curl, CURLOPT_USERAGENT, "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0");
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, "email=test&pass=test");
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
curl_easy_setopt(curl, CURLOPT_COOKIEJAR, "cookies.txt");
curl_easy_setopt(curl, CURLOPT_COOKIEFILE, "cookies.txt");
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, false);
curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, false);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &response);
curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, true);
curl_easy_setopt(curl, CURLOPT_VERBOSE, true);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
And here's the WriteCallback:
size_t WriteCallback(void* ptr, size_t size, size_t count, void* stream)
{
((std::string*)stream)->append((char*)ptr, 0, size * count);
return size * count;
}
When I send a request, the fields are filling successfully, but the form is not sending. I'm not redirecting to another page.
Thank you in advance.