1. Select the Mapped Key Wizard from the Wizards menu.
2. Select the Combat Key Map to put your new mapping into.
3. Select "Straight Key Broadcast Plus Click" as the type of Mapped Key to set up
4. In the final page pick the Hotkey that initiates the spell, and tell it where to go

If you want to initiate it by pressing a different key than it sends, go into the top left pane under Key Maps, then select the new mapping in the bottom left pane to set its Hotkey.

p.s. this is also mentioned in the Quick Start Guides, among other places http://isboxer.com/wiki/WoW:Quicker_...e_is_no_part_8