autocast for spelss

How can I give autocast to chainlighting of other spells?

Give the unit chain lightning but have it NOT have any icon for use and random hotkey, and then make a slow based ability that lasts 0.1 secs with no art and the SAME OR LESS COOLDOWN as chainlightning. Have a trigger:

A hero learns Chain Lightning, give unit slow.

Then:

A unit casts Slow, order unit to cast chain lightning at (casted at unit).