The prima facie case for free will* is that it feels free. If you can predict the action before the feeling it removes that argument (unless you want to invoke time travel as an option)

*one of the predominant characterisations of free will, anyway. I'm a compatiblist, so I have no issue with caused feelings of decision making being in conflict with free will. I also have a variation of Tourette's, so I have a different perception of doing things wilfully when compared to most people. It's really hard to describe how sometimes you can't tell if something was tic or not.

Well, what does freewill mean to scientists?

That it precedes voluntary acts tells us that most of what we do are not conscious. Which has been known for over a century, maybe millenia.

(opinion stolen from some Chomsky video)