I have a correct manifest.json file required to prompt chrome to ask user to install the native app (Add to home screen) and it works in mobile chrome. (https://developers.google.com/web/fundamentals/app-install-banners/native)
And I know this functionality was built by chrome for chrome only, I just want to know can we somehow get this functionality working in Facebook inapp Browser - by maybe firing some event or does Facebook provide some native functionality to do so ?
If i understood your question, you are trying to use Smart App Banners in the Facebook in-app browser.
You can only do that when the browser has this feature implemented. To my knowledge, this is the case for chrome & Safari but not for the Facebook embedded browser.
You will normally need to create a custom Smart App Banners by yourself on your web site and to use Facebook App Links to make a deep link which will open your app. See : https://developers.facebook.com/docs/applinks/
There are some drawbacks. For example, if the app is installed it will open, but if it is not installed, the user will fallback to the store to download it without being transferred to the content he wanted to see after the download (deferred deep link is not supported).
A good article : https://blog.branch.io/how-to-deep-linking-from-facebook/
