Hi Nirmal,
I think this widget can do what you want: https://appstore.home.mendix.com/link/app/67293/
It allows you to display the button like you want, and trigger a microflow (in which you can show your notifications screen for instance) when clicked.
Hi,
Toastr is a good and relatively easy to use with Mendix although the actual look and feel might be a bit different.
https://appstore.home.mendix.com/link/app/45837/AuraQ-Limited/toastr-Notifications
-Andrej
I think the tool used on Mendix homepage is a third party API called intercom