Counter target spell, then proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
If the spell is an illegal target when Fuel for the Cause tries to resolve, it won’t resolve. You won’t proliferate.
You must be able to target another spell in order to cast Fuel for the Cause. You can’t just cast it in order to proliferate, and it can’t target itself.