WoW Quick Start Guide: Quickly add many hotkeys, optionally with auto-assist -- in this case, you dont want auto-assist. just add 'S' I suppose and send it to All w/ Current. Most people would put this particular key in the General key map.

After following that and exporting, pressing S will broadcast BUT it won't hold it down while you hold S down, it will just fire off S once when you press it (or release it actually). So because this is a movement key, you will need to change an option on the Mapped Key it created.
1. In the top left pane, under Key Maps, select the Key Map you added "S" to
2. In the bottom left pane, under Mapped Keys, select the Mapped Key you added (S)
3. In the bottom right pane, find the check box that says Hold keystrokes while the hotkey is held
4. Export to Inner Space

S now broadcasts to all and works for movement, unless Key Maps are disabled with Ctrl+S.

Tip: You can also set a modifier for this, like Ctrl+S. Then S doesn't automatically broadcast if you're typing. Up to you.