Trapping SC_SCREENSAVE when app has no focus

I need to trap the WM_COMMAND/SC_SCREENSAVE message to detect when a screen saver will launch.

The following code will work fine, but one when the application has the focus:

procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;
  var Handled: Boolean);
begin
  Handled := FALSE;
  case Msg.Message of
    WM_SYSCOMMAND :
      if Msg.wParam = SC_SCREENSAVE then
        Handled := TRUE; // disable startup of screensavers
  end;
end;

But how can I trap this message when the application
does not have the focus?