[A2HS][Add to Home Screen] 'beforeinstallprompt' not firing in Android Web View

I implement A2HS to my website according to A2HS

Full supported platform

It worked on all supported platforms except "Android webview". 'beforeinstallprompt' not firing in Android Web View.

However, the "Add to Home Screen" option still display on the setting of Webview. Android Web View setting

I found out although the website has NOT implemented A2HS yet, the Android Webview still displays "Add to Home Screen" option, such as: google.com

Finally, if beforeinstallprompt' event is not available, alternatively could we use a native function which links a button in website with "Add to Home Screen" in Android Webview ?. Notice that, I only make website, and users can open my website url by Android Webview via every app like FB msg, email...