How to send base64+ c++ post request parameters using curlcpp?

Question

C++ / C#

How to send base64 post request?
If you just send
string.c_str ()
then everything will not work exactly.

Yes
- const string BASE_64 = "what without code";
- Library curpcpp

Here is the code, what needs to be fixed?
Help me please!

CURL* curl;
  CURLcode res;

  curl_global_init(CURL_GLOBAL_ALL);

  curl = curl_easy_init();

  if (curl) {

    curl_easy_setopt(curl, CURLOPT_URL, URL_in.c_str());
    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, DATA_POST.c_str());
    res = curl_easy_perform(curl);	

    if (res != CURLE_OK) {
      fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
    }

    curl_easy_cleanup(curl);

  


  }

  curl_global_cleanup();

AND ANOTHER QUESTION HOW TO SEND ANOTHER ADDITIONAL. PARAMETERS TOGETHER WITH base64
Thank you very much)