Lua Addon/Macro question

[kerovi]

Hello everyone, I need some help here.
I'm trying to make something like this: Lets say on event SPELL_CAST_SUCCESS I want myself to cast counterspell which is lets say spellID = 11 when target casts flash light which is lets say spellID = 20
It would be something like this in Lua if target:GetSpellId() == 20 then CastSpellByName("Counterspell") < But I'm not sure how it is exactly (I also got that Lua protection removal so I can use protected Lua functions)

I would really appreciate if someone could make short working version of this. Thanks in advance.


Summarized info:
I want myself to cast counterspell when target casts flash light