Russel's teapot.
If I claim Russel's teapot doesn't exist, the burden of proof is on you to prove it exists if you disagree with me, not me, because the set of claims that are so poorly defined and unlikely as to make it an unjustified burden to expect anyone to caveat the claims to make them technically correct is infinite - someone who makes an extraordinary claim without being able to provide a justification therefore needs to be prepared to take on the burden of proof.
I equally claim that santa doesn't exist, and yet will insist the burden of anyone disagreeing with my claim lies on them, not on me, even if I make the initial claim, for exactly that reason.
In this case the claim that free will does not exists puts the burden on those who disagree *because nobody here or otherwise* have presented a definition of free will that most people would agree to, not least because without narrowing the scope with a definition I have the freedom to apply whichever definition I prefer, and I have several places in this thread.
E.g. I'd argue that "free will" would require an entity to be able to do something which cannot be explained by the combination of determinism and randomness. Given we're unaware of any kind of processes that can produce outputs that are not either deterministic or random modulated by some sort of probabilistic distribution, to claim that a process that can produce such an output exist is an extraordinary claim that would give you a Nobel in physics if you could demonstrate it.
As such: Russel's teapot.
I'd love to be wrong. I'd love for someone here to come up with a definition of free will that overcomes that problem, as if so I'd be witnessing a revolution in philosophy surpassing anything we've seen in centuries.
Just like I'd be delighted to be wrong when I say that santa doesn't exist, but I do believe I'm far more likely to be proven wrong about santa than free will, given that people can at least come up with definitions of santa that are somewhat coherent if unlikely.