Kant examines the will and presents his argument for how the will is free. Oedipus Rex and Hamle the characters caught in situations, which they would not choose willingly. From Kant's understanding of the will, how are these plays an understanding of how the will may act in freedom.
You could examine how the will of others impacted both Hamlet and Oedipus. While Kant did examine free will, sometimes the will of others outweighed that of the main character in order to set up the action.
Hamlet and Oedipus are both influenced by supernatural forces. In each case, they get news from a messenger. I think you could argue that the actions they took would have been quite different if they had not known this information. The messenger affected their free will.