code: function onMessageReceivedSubscribe() {
      /*
        If you're integrating amp-web-push with an existing service worker, use your
        existing subscription code. The subscribe() call below is only present to
        demonstrate its proper location. The 'fake-demo-key' value will not work.
        If you're setting up your own service worker, you'll need to:
          - Generate a VAPID key (see:
            https://developers.google.com/web/updates/2016/07/web-push-interop-wins)
          - Using urlBase64ToUint8Array() from
            https://github.com/web-push-libs/web-push, convert the VAPID key to a
            UInt8 array and supply it to applicationServerKey
       */
      self.registration.pushManager.subscribe({
          userVisibleOnly: true,
          applicationServerKey: convertedVapidKey,
      }).then(pushSubscription => {
          persistSubscriptionLocally(pushSubscription);
          broadcastReply(WorkerMessengerCommand.AMP_SUBSCRIBE, null);
      });
  }
docs: Subscribes the visitor to push.
  The broadcast value is null (not used in the AMP page).