Chrome on Androids offers it automatically for PWAs that meet some requirements. You can also add it manually from the browser menu.

https://vgy.me/GbvnOM.png

https://developers.google.com/web/fundamentals/app-install-b...

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.

Scroll all the way to the top then a link will appear in a bottom toolbar type thing.