code: @Nullable
    public static PasswordAuthentication requestProxyPasswordAuthentication (@Nullable final String sHostName,
                                                                             @Nullable final int nPort,
                                                                             @Nullable final String sProtocol)
    {
      return requestPasswordAuthentication (sHostName,
                                            (InetAddress) null,
                                            nPort,
                                            sProtocol,
                                            (String) null,
                                            (String) null,
                                            (URL) null,
                                            RequestorType.PROXY);
    }
docs: Shortcut method for requesting proxy password authentication. This method
  can also be used, if this class is NOT the default Authenticator.
  
  @param sHostName
  Hostname to query
  @param nPort
  Port to query
  @param sProtocol
  Protocol to use
  @return <code>null</code> if nothing is found.