If you have a valid SMU email address that is currently active and you know the password to it, then here's how you can find your s-number.
1. Visit https://smu.ca/smuit
2. Click on Sign in at the top right corner
3. Sign in with your SMU email address and password. Authenticate your account if asked.
4. After signing in, your name should appear in the top right corner. Click on your name, and then click on My Profile.
5. Your s-number will be listed under Organizational ID.