Will for Kant is the ability to act in accordance with a conception of law. In spite of his confusing rhetoric about free will, this is clearly not the voluntarist notion of a faculty superior to reason, free to do or choose any arbitrary thing. However much I dislike images of law in ethics — which by default suggest what Hegel called “positive” or empirically existing, first-order law — acting in accordance with a conception of law is clearly not acting arbitrarily.
Kant also distinguishes between acting in accordance with a conception of law from merely acting in accordance with law. The latter would be mere obedience, without thought. So the important thing is not really the law as such, but thought about how to interpret it. (See also Kant’s Groundwork; Kantian Freedom; The Autonomy of Reason.)