You cannot add buffs to units via triggers, you have to spawn a dummy unit that will cast a spell that will give buff to the targeted unit.
Here's a test map I made. Hit escape key to stun the footman for 5 seconds, you can modify the 'Custom Storm Bolt' ability to increase the stun, or use other abilities that can add buff to units, and make sure target abilities should have a cast range and missile speed set to 99999. (Use the test map below as a reference).
Does this mean I'd only be able to base 1 ability off of slow buff or ability? What if i'm making a whole lot of custom abilities and buff tooltips and I need to base it off default slow many times?Use 'slow' ability, customize it, increase the range and speed like I said, change the buff of the ability from 'Slow - Buff' to 'Slow Aura - Buff' (You can create your own custom slow buff or modify the default slow buff), then order the dummy unit to cast the 'Slow Ability' (Target Unit Being Cast), not 'Attack' (Target Unit Being Cast).
Oh okay so I have to base my custom ability as the same thing as the dummy unit's. I understand now thank you!Make more dummy units with different abilities, and with different buffs on each ability.
Edit: Here, made you two dummy units, each with different abilities and each ability gives different buff, one is stun buff and the other is a slow buff. Hit left key to slow the footman, hit right key to stun the footman.