/hotbutton buttonname
Click it on your hotkey bar. Right click it and enter a command in the text box.
(It seems to me there is a way to include the /command in the initial creation, but I never did it that way, so I dunno for sure.)
http://www.jaburt.com/eq/eq_commands.htm will give you boatloads of commands... some apply to EQEmu, some don't; but it's a pretty exhaustive list!