I tried like this, but when I access the menu, the Preferences item is not enabled :

class Application(toga.App):
  def startup(self):
    # ... build UI here
    self.main_window = toga.MainWindow(title=self.formal_name)
    self.main_window.content = MainBox
    self.main_window.show()  
    
  def preferences(self):
    # Create a new window for the preferences
    pref_window = toga.Window(title="Preferences")
        
    # Create a preferences window layout
    pref_box = toga.Box(style=Pack(direction=COLUMN, padding=10))

    # Example preferences settings (could be text inputs, toggles, etc.)
    theme_label = toga.Label("Choose Theme:", style=Pack(padding=(0, 5)))
    theme_selection = toga.Selection(items=["Light", "Dark"])

    notification_label = toga.Label("Enable Notifications:", style=Pack(padding=(0, 5)))
    notification_toggle = toga.Switch()

    # Add widgets to the box
    pref_box.add(theme_label, theme_selection, notification_label, notification_toggle)

    # Set the content of the preferences window
    pref_window.content = pref_box
    pref_window.show()