@Linda-MacDonald by default it automatically appears the first time your user visit it on mobile.
Here is an advanced solution if you want to show a custom add button :
https://stackoverflow.com/questions/50332119/is-it-possible-to-make-an-in-app-button-that-triggers-the-pwa-add-to-home-scree
But you might need to add PWA by yourself to do it.