@gianfranco74 Seems I don’t quite follow your dots in this issue, however, overall maybe the issue is somewhere else ? Try checking this and this, maybe things will clear up a bit for you.
Otherwise, I’m not sure if this will work for you but it was ok for WPF custom windows:
var someDialog = new Controls.SomeDialog();
someDialog.ShowInTaskbar = false;
new System.Windows.Interop.WindowInteropHelper(someDialog).Owner = Rhino.RhinoApp.MainWindowHandle();
//ShowDialog or sth else suitable for you
However, this does not solve anything for Mac.