HOW TO GENERATE ACCESS CODES FOR SOME CONTENTS

Please i urgently need help regarding this.
I installed wp quiz on my site so that candidates can get there and test their knowledge but those are the questions they are gonna meet in the real exam. So i want to generate access codes to the quiz page so that every candidate will only be able to access the quiz if he/she has entered a correct access code. I know about some plugins that can help restrict contents, they are not perfect for me.. I want every candidate to get access code from me, different access code for each candidate or user. I will be very glad if someone is able to help me on how to do this task.