Your CSU email address has already been loaded into codePost, so you can complete the registration process by going to the login page and clicking on the "Forgot password" link. Enter in your CSU email address (including the "students." part) and then check your email. Sometimes it takes more than one try, so please work on this well before the deadline.
If you added the class late, then you can get access to the codePost course for this class by going to https://codepost.io/signup/join?code=GZ5KEDBN6H. (If you already have a codePost.io account, then use the same link to just get access to the codePost Course for this class.) Remember to use your CSU email address (including "students.").