More generally, xfce uses the common method of invoking the command using the name of the associated binary. On my system there are a bunch of them, including xfce4-popup-whiskermenu in usr/bin/ You can find it on your system by poking around or using
locate xfce4-popup-whiskermenu
on a command line.
They all seem have man pages so, for example, you can find out about the usage of the xfce4-popup-whiskermenu with the command
man xfce4-popup-whiskermenu
You are not limited to binding only xfce commands to keystrokes, and can start any program using the same command you would use in a terminal.