CString edata =  CreateUrlEncodedString( data ).c_str();//обертка для InternetCreateUrl
                                                        //строка param1=1&param2=2&param3=русский становится param1=1&param2=2&param3=%F1%F2...
CString headers = _T("Content-Type: application/x-www-form-urlencoded\r\n\Content-Transfer-Encoding: 8bit;");//заголовки

        char* opt_str = 0;
#ifdef UNICODE
        USES_CONVERSION;
        CStringA ansi_str = W2A(edata);
        opt_str = ansi_str.GetBuffer();
#else
        opt_str = edata.GetBuffer();
#endif

        bSend= HttpSendRequest(hRequest.GetObject(), headers,headers.GetLength(), (LPVOID)opt_str, edata.GetLength() );

S>CString headers = _T("Content-Type: application/x-www-form-urlencoded\r\n\Content-Transfer-Encoding: 8bit;");//заголовки
S>

CString headers = _T("Content-Type: application/x-www-form-urlencoded; charset=utf-8");

Content-Type: multipart/form-data; boundary=---------------------------182832622229331
Content-Length: 1221

...
-----------------------------182832622229331
Content-Disposition: form-data; name="subject"

русский
-----------------------------182832622229331

...

CStringA UTF16toUTF8(const CStringW& utf16)
{
    CStringA utf8;
    int len = WideCharToMultiByte(CP_UTF8, 0, utf16, -1, NULL, 0, 0, 0);
    if (len>1)
    { 
        char *ptr = utf8.GetBuffer(len-1);
        if (ptr) WideCharToMultiByte(CP_UTF8, 0, utf16, -1, ptr, len, 0, 0);
        utf8.ReleaseBuffer();
    }
    return utf8;
}