Go to this link:
select the time zone you would like and also date and time to give exam and complete the details, if you are working for company give the details of your manager who would be paying invoice or you could give your own details and Mendix will send Invoice.
In general first you need to do the RAD course, either online or instructor led. After you can go for certification. If you do a classroom course, the certification will be included. If you do the online path, you can request for the certification separately. You'd either have to provide your company details or your own.
Please refer to: https://academy.mendix.com/link/certifications/23/rapid
For any additional questions, please contact firstname.lastname@example.org