Currently we have HTTPS only setting enabled for our web app. But for security reasons we want to completely hide port 80. How is it possible?
So the online port scanners would return only 443 as available?
You can't do this directly from the WebApp. Azure WebApp is a PAAS service not IAAS. You don't have to worry about this infrastructure security issue; Azure takes care of it.
A real security enhancement will be to place your WebApp behind a network service - such as App Gateway or Front Door - which will hide port 80 on the WebApp but more importantly add a network security layer to your infrastructure.