I assume you have enrolled a particular certification event as published here: https://academy.mendix.com/link/certification/22/intermediate
In that case, you should have had a confirmation email of your registration. Which tells you that you will get final details once registration has been accepted.
If you havent received that direct confirmation, either the registration didn’t finish correctly or the mail ended in your spam box.
Also, the intermediate exam is online, but not self-paced. Its a proctored exam at a specific moment with a timelimit. So the moment of the exam is at the moment of your choosing