Going directly to the chase.
.... I can't move my finger at "will".
As you wrote, you can't will your finger to move. For one's 'will' to do so would mean that self same 'will' organize the muscles, bones, ligaments, tendons, etc., in the local area the finger to move in a particular way. Most of those components are trained through practice and maturation to some number of coordinated operations over time.
The 'will', given conditions are proper might select one of these operations. But the particular operation would conform to existing situations in which the finger and the hand find themselves. So its not willing the finger to move that would be a demonstration of will. It must, to be will, be an explicit and exact finger movement for such an activity to have any hope of being determined to be and act 'of free will'.
So no, you can't say I can move my finger at 'will'. You can't because it's nearly impossible to set up conditions whereby such an action would be a clear demonstration of anything resembling 'free will'.