From experience:
1. You receive the test invitation email with instructions.
2. You send them an email on the one provided in the instructions (Outside Canada for the Subject of the email and your full name, application number, UCI, and exam from-to date range for the Body of the email).
3. Send the email and wait 24-48 hours.
4. Someone will send you an email acknowledging your situation - make sure to check your spam/junk folder.
5. You can now do the test by clicking on the link that they provided in their invitation email.
I hope this helps.