It is certainly true that to the best of our sensibilities, we seem to have something like a free will. We can, for example, take any kind of data that we are exposed to in the present, and make what we believe to be free willed decision. I can concede that this appearance might very well be just an illusion, but if it were ever the case that we could somehow become aware that we were not making a free willed decision, then that illusion would disappear as well. Simply providing data to a person and allowing them to independently make what they think is a free-willed decision is not something that would dissolve such an illusion. Further, if free will does not really exist, then it should be possible to contrive a hypothetical situation where all of the decisions that someone will make can be anticipated before they occur.
If, however, it were ever allegedly possible to predict with perfect certainty what answer a person would give to a specific question, but the person had already decided to say that their answer would be the opposite of whatever their alleged predicted answer would be, then the prediction will always be wrong, so no mechanism for prediction such a decision exist, showing in turn that free will exists. If, however, they were not capable of deciding to do that, then they would not have any illusion of free will either. The illusion persists, however, so free will must also exist.