How to get this discount ?

Hi, every superheroes.
In another forum, I saw someone got a -90% discount coupon for G coins 2 days ago.
Here's the picture


Many people asked him how to get it, but he didn't know either.
He thought it was one of the civil war gift and everyone had it.
So I just want to ask you guys if you know about it.
Thanks for the answer, btw.