Thanks! As far as I'm aware, you can't manually trigger the 'add to homescreen' prompt as it's a system-level interaction. Android Chrome & Firefox should trigger it, but if not it's under the default browser settings dropdown on Android, and under 'share' (I think) for IOS.
https://vgy.me/GbvnOM.png
https://developers.google.com/web/fundamentals/app-install-b...