Test different email addresses
The recruiter log-in window will give you immediate feedback on whether the email address is registered in the system or not. If you have only a few email addresses you use all the time, it might be worth to try every single one.
If your email address is not registered, after clicking "Next" you will not be asked to enter a password, but you will see the message "Welcome, you don't have an account associated with this institution yet. Click on the button below to create one."
If your email address is registered in the system, you will see the message "You have a recruiter account associated with this institution" and the password field appears.
Reach out to your university contacts
If you cannot find your account with the method above and have already posted a job ad, your university contact will be able to find your job ad in the system (with the title or company name) and see the "Recruiter Contact Information", which also contains the associated email address.