Xfce is a desktop environment, like Lxde (which is what you are using since
it's the default in Trisquel Mini).
Any desktop environment including Xfce, Gnome (the default in Trisquel 7),
MATE (the default in Trisquel 8), and Lxde should have a menu for setting
keybindings. However, it may be called something slightly different and in a
slightly different place, which is why those of us who don't use Lxde can't
tell you exactly where to look.
In MATE, the menu is called 'Keyboard Shortcuts'. It might be called that in
Lxde, or it might be called something else like 'Keybindings'. If you've
installed Synapse, open Synapse, type 'key', press down arrow, and see if you
can find it.
Once you find that menu, map the command xkill to whatever keyboard shortcut
you want. I have it mapped to Ctrl+Alt+X.
Don't feel self-conscious about having to ask questions. Stepping outside
your comfort zone to protect your freedom is nothing to be ashamed of.
