Minus hp/mana per attack, is that possible with GUI

I am trying to make an ability that whenever u attack, it triggers something. I cant seem to find an even trigger for that. Is there 1? At the moment i just need it to minus mana per attack made.

Weird i cant find the set life of life value - something.

You would find it under Unit - Set Unit Property

Event: A unit attacks
Condition: Your Conditions Here
Actions: Unit - Set unit mana to Current Mana of Attacked Unit - <Your reduction rate here> OR if you want mana drain on attacking unit per hit
Set unit mana to Current Mana of Attacking Unit - <Your reduction rate here>
OR if hp
Set unit health to Current Mana of Attacked Unit - <Your reduction rate here>
OR is health drain for attacking
Set unit mana to Current Mana of Attacking Unit - <Your reduction rate here>

Firstly i cant even find the event a unit attacks.

Also, where u get those actions and how u make it minus.
Is it the Unit- Set mana (To Value) trigger??

i am using WE btw

Actually no, you can barely make it subtract mana whenever the unit BEGINS attacking.

And the actual trigger would be:

For Value of mana you begin with arithmetic, in arithmetic you choose mana of attacking unit for first one and then minus and then the quantity you want to subtract.

I see,thanks for the help.