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This is not working like title said, it minimized to open taskbar not tray, so the button "x" is changed to "_", is it possible to change it?

I'm sorry the addon doesn't do what you want, but it does exactly what it's supposed to do.

https://www.computerhope.com/jargon/t/taskbar.htm

What you mean is called "system tray" and it is a subset of the taskbar.

I have no plans on implementing this, but the add-on is open source and you're free to modify it however you want to.