According to the google developers website, to show the installation banner the website must...

Be visited by the user twice, over two separate days during the course of two weeks.

You basically have to refresh the app after the login page is loaded.

Desktop users only: have a look at the google developers website to learn how to simulate add homescreen events.

After tapping the phone icon the official banner will pop up

It will take a few seconds before the installation finishes

Click open to start using this PWA

And there you have it!

Force the Add to Home screen event

There is always a plan B

What now?

After having the PWA installed you will have a mix of web/native experience