For those who like me are new and may need help with certain duty actions that you'd like to have bound to a hotbar key: select SYSTEM - KEYBIND - HOTBAR and go all the way to the bottom where you can enter the keybind you need and then click APPLY to make it stick.

Since I started WoW in 2005, I've used my mouse for travel and my number keys to fire off skills. I know a lot of folks use the left hand for travel and the mouse for skills, so they might not see a need to bind a key for a duty action. When you have to run around a lot AND click the duty action button with the same mouse ('Return of the Bull' for example) it gets very hard to keep up. I'm grateful to SE for giving us so many control and interface options.