Solved, here's the code if someone needs it:

Code:
// set URL
URI uri("http://mysite.com/karas/login.php?user=karas&pass=blah");
string path(uri.getPathAndQuery());
if (path.empty())
  path = "/";

// set connection
HTTPClientSession session(uri.getHost(), uri.getPort());
HTTPRequest req(HTTPRequest::HTTP_POST, path, HTTPMessage::HTTP_1_1);
session.sendRequest(req);

// print response to STDOUT
HTTPResponse resp;
istream& rs =  session.receiveResponse(resp);
StreamCopier::copyStream(rs, cout);
Instead of C strings class string is used beacuse of references.