MainWindow.blp

using Gtk 4.0;
using Adw 1;

template $ExampleWindow: Adw.ApplicationWindow {
  title: 'Python - PyGObject - GTK';
  default-width: 683;
  default-height: 384;

  content: Adw.ToastOverlay {
    child: Adw.ToolbarView {
      [top]
      Adw.HeaderBar header_bar {
        [end]
        MenuButton {
          icon-name: 'open-menu-symbolic';
          menu-model: primary_menu;
        }
      }

      content: Gtk.Box {
        orientation: vertical;
        spacing: 12;
        margin-top: 12;
        margin-end: 12;
        margin-bottom: 12;
        margin-start: 12;

        Gtk.Label {
          label: 'Access the menu and click on the about button';
          vexpand: true;
        }
      };
    };
  };
}

menu primary_menu {
  section {
    item {
      label: _('Preferences');
      action: 'app.preferences';
    }

    item {
      label: _('About');
      action: 'app.about';
    }
  }
}