As far as I know, the building blocks of Kant's moral philosophy include:
Kant's epistemology, thinkability(what?) of the free will.
Categorical imperative (universality, treat things as ends not means)
The Categorical Imperative only gives us the criteria to check whether an act has moral worth (whether X is the right thing). However, I fail to find where Kant addresses the motivation or reason to obey the categorical imperative (Why we ought to do the right thing).
Or we can simply put the argument in the following way: If you have the will to be a free person, you are acting according to a law autonomously, and this law includes the categorical imperative, then as a result, your motivation to act morally is simply that you want to be a free person.