If your brain made decisions outside of your free will which you become conscious of after the fact, then that would manifest itself as a frequent conflict: we would often disagree with the action that was taken. Because we don't disagree, it must be something that is integrated with will.

Concretely, we are not surprised that our finger moved; we believe we wanted to do that and we agree with that action.

Moreover, this readiness potential phenomenon works on short time scales. The will operates on long time scales. I can plan at 11:55 that I will move my finger at 12:00, five minutes ahead. And then when the time comes, do just that. Still, the readiness potential will play out the same way: the commands to move the finger precede the conscious awareness of the finger moving.