Minimal gnome hello world using glade + gjs

hello.glade

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.18.3 -->
<interface>
  <requires lib="gtk+" version="3.12"/>
  <template class="Gjs_AppWindow" parent="GtkApplicationWindow">
    <property name="can_focus">False</property>
    <property name="title" translatable="yes">gnome-hello</property>
    <property name="window_position">center</property>
    <property name="default_width">200</property>
    <property name="default_height">100</property>
    <child>
      <object class="GtkBox" id="box">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkLabel" id="label">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
          </object>
          <packing>
            <property name="expand">True</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkButton" id="button">
            <property name="label" translatable="yes">Close</property>
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
          </packing>
        </child>
      </object>
    </child>
  </template>
</interface>

hello.js

const Lang = imports.lang;
const GLib = imports.gi.GLib;
const Gio = imports.gi.Gio;
const Gtk = imports.gi.Gtk;

function readFile(filename) {
  let file = Gio.file_new_for_path(filename);
  let [success, data, length] = file.load_contents(null);
  return data;
}

const App = Lang.Class({
  Name: 'App',
  Extends: Gtk.Application,

  _init: function() {
    this.parent();
  },

  vfunc_activate: function() {
    this.window = new AppWindow({application: this});
    this.window.present();
  }
});

const AppWindow = Lang.Class({
  Name: 'AppWindow',
  Extends: Gtk.ApplicationWindow,
  Template: readFile('hello.glade'),
  Children: ['label', 'button'],

  _init: function(params) {
    this.parent(params);
    this.label.set_text('Hello, World!');
    this.button.connect('clicked', this.onClose.bind(this));
  },

  onClose: function() {
    this.application.quit();
  }
});

let app = new App();
app.run(ARGV);

If GTK 4.0 is present on the system
imports.gi.versions.Gtk = "3.0";

I'm using glade-3.38.2. I don't see a template option in glade so where/how can the <template ....> be added?