WhatsApp Desktop application, how to reduce to icon?

You want to minimize the application to system tray.

I do that for many programs that I have including Whatsapp.

You need to install a program that supports that feature.

I use "Actual Window minimizer".

https://www.actualtools.com/windowminimizer/

It is not a free program. But you can find free programs that can have this feature.

You can do a search and you can try the free one in the below link that I did test on it last year and works fine.

https://www.4t-niagara.com/tray.html

I never tested it on Windows 10 but I was using it on Windows 8.

You can search for similar apps in the below link.

https://alternativeto.net/software/4t-tray-minimizer/?p=3