What you're looking for is easy to do,
Finallegacy. The example below deals damage equal to the caster's Strength, including bonuses.
[TRIGGER=GUI Trigger]Stat Damage
Events
Unit - A unit Starts the effect of an ability
Conditions
(Ability being cast) Equal to (Your Ability)
Actions
Unit - Cause (Casting unit) to damage (Target unit of ability being cast), dealing (Real((Strength of (Casting unit) (Include bonuses)))) damage of attack type Spells and damage type Normal
[/TRIGGER]
To get to the event, search for
Unit - Generic Unit Event in the list.
The action is a bit more complicated. I use the
Unit - Damage Target action.
Explanation
(Casting Unit), which is the caster, will deal damage to (Target Unit Of Ability Being Cast), which is the target.
NOTE: Make sure it's TARGET UNIT OF ABILITY BEING CAST, as TARGETED UNIT will not work!
Now, to get it to deal the Hero's Strength, we have to use
Conversion - Convert Integer to Real.
Search for
Hero - Hero Attribute, select the attribute, which unit, and finally decide whenever you want it to include bonuses or not.
If you need further explanation, do not hesitate to ask!
Happy mapping!