Download the app, allow push notifications when prompted, wait for the push notification (usually within a minute) and then copy/paste it in to your basket when you're ready to checkout.
If you missed the prompt to allow push notifications, don't worry, you can go into your phone's settings > apps > allow push notifications, clicking on the toggle for this.
The app discount is valid for first time app users opted in to push notifications and on orders over £30 only.