Ok, thanks for your efforts, guys, I found the solution, i should have removed 'http://' from the URL... that's it :) if anybody is interested, here is the code :
int main()
{
LPCWSTR host = _T("145.239.244.171");
LPCWSTR url = _T("api/get/customers");
HINTERNET hIntOpen = InternetOpen(_T("Mozilla/5.0 (Windows NT 6.1; rv:58.0) Gecko/20100101 Firefox/58.0"), INTERNET_OPEN_TYPE_PRECONFIG, NULL, NULL, 0);
HINTERNET hIntConnect = InternetConnect(hIntOpen, host, 3001, NULL, NULL, INTERNET_SERVICE_HTTP, INTERNET_FLAG_KEEP_CONNECTION | INTERNET_FLAG_NO_CACHE_WRITE, 0);
HINTERNET hHttpOpenReq = HttpOpenRequest(hIntConnect, _T("GET"), url, NULL, NULL, 0, INTERNET_FLAG_RELOAD, 0);
BOOL hHttpSendReq = HttpSendRequest(hHttpOpenReq, NULL, NULL, 0, 0);