https://gist.github.com/Scrumplex/8f528c1f63b5f4bfabe14b0804adaba7
xfce by default uses https://packages.gentoo.org/packages/gnome-extra/polkit-gnome
If not automatically started, you can manually start it (as a user, not root):
/usr/libexec/polkit-gnome-authentication-agent-1
