Alt? Just Alt? Not like ... Alt+1 through Alt+=? Generally you don't want to broadcast "just" the modifier...

The information most people need is this WoW Quick Start Guide section: Quickly add many broadcasting hotkeys, optionally with auto-assist, and most people would add various hotkeys like Alt+1 through Alt+= rather than just Alt. If you want Alt to broadcast for a different purpose/reason, explain what you're trying to accomplish with Alt and I might have a different answer for you