Scan the QR code with your phone or open the link directly. Then you can install the web app as an app on supported devices.
On Android/Chrome a real install button usually appears. On iPhone in Safari: Share → Add to Home Screen.
Android
Open the link → browser menu or install button → install.
iPhone
Open in Safari → share icon → Add to Home Screen.
Installation QR code
This QR code opens the install page of the PWA.