Using `wmctrl` and `skip_taskbar to remove an application from the Alt-Tab switcher works for Firefox and Google Chrome but not for CopyQ

Question

Based on the information at Remove app from alt-tab switcher I was able to successfully prevent Google Chrome and Firefox from appearing when I press Alt-Tab by using...

1. wmctrl -r "Google Chrome" -b add,skip_taskbar, and
2. wmctrl -r "Firefox" -b add,skip_taskbar, but
3. inexplicably (to me) wmctrl -r "CopyQ" -b add,skip_taskbar does not prevent CopyQ from appearing when I press Alt-Tab.

How can I prevent CopyQ from appearing when I press Alt-Tab?