Free will is a construct contingent on a brain or neurons.
It does not come from nothing, and therefore it does have a cause.
Free will works by intentions.
If we intend to do something and do it, it is therefore free will.
If we intend to not do something and don't do it, it is also free will.
Merely acting on our intentions is a demonstration of free will, and the fact that all actions we intend to do are contingent on the brain/neurons shows that free will has a cause/origin.