class HttpClient
{
public:
...
HttpClient&  ProxyAuth(const char * User, const char * Password)
  { proxy_user = User; proxy_password = Password; return *this; }

String       proxy_user;
String       proxy_password;
...
};

String HttpClient::Execute(Gate2<int, int> progress)
{
... if(std_headers) { ...

if(use_proxy && !proxy_user.IsEmpty())
{
  request << "Proxy-Authorization: Basic "
    << Base64Encode(proxy_user + ':' + proxy_password)
    << "\r\n";
}
...}...
}