In order to register for the exam you must first submit all of your test application approval paperwork to PSI, and be approved to take the exam.
You cannot register for the exam until your test application paperwork (test application, CORI form, references, education certificate, etc.) has been approved by PSI. You must finish the entire course in order to receive your education certificate and apply to take the exam.
Not sure what you need to submit PSI to take the exam? This article goes over the basics, and we cover the test application process in depth in the Massachusetts Real Estate Test class.