IV Kant doesn t deal with this question in the Critique in a way that he does, at least implicitly, in the Groundwork. There the idea seems to be that the alternative we are supposing, that a principle might simply be a practical law, quite independently of whether a free agent could legislate or will it, is unacceptable because it implies heteronomy and is inconsistent with autonomy, the property the will has of being a law to itself. Now there is a weak sense in which the very idea of practical law presupposes autonomy, since, by their very nature, practical laws bind any rational agent, that is, any being with a will. So it is the will itself that is the ground of obligation. Nothing our norm-egoist thinks, however, need be inconsistent with that.
Free agency is a formal rather than a material or substantive capacity.
Access to the practical law can only come through the exercise of free will, and this requires that practical law must be capable of being legislated by each will.