How can I Change the shape of Firemonkey windows?

In VCL I can change the shape of a form using SetWindowRgn.  I would like to do the same with a FireMonkey App when it is on the Windows platform.  I would also like to do it on macOS as well, but that may be asking a bit much.

The code I tried compiles, but has no discernible effect:

procedure TfrmMain.FormCreate(Sender: TObject);
{$IFDEF MSWINDOWS}
var
  regn: HRGN;
{$ENDIF}
begin
  {$IFDEF MSWINDOWS}
  regn := CreateRoundRectRgn(0, 0, ClientWidth, ClientHeight, 40, 40);
  SetWindowRgn(WindowHandleToPlatform(Self.Handle).Wnd, regn, True);
  {$ENDIF}
end;

Note that I have already turned the borders off with BorderStyle = None in the Object Inspector.

Is there another way of shaping forms in FireMonkey?  Could the other controls on the form (e.g. a toolbar) interfere with the rounded corners.