You can sign up both before or on the day of the event:
Prior Sign Up
You can visit the University's Unibuddy page where you can sign up for the platform beforehand, ensuring that you have login details prior to the live event.
You can find this page by a simple Google search. You can usually type the university's name followed by "chat with us." For example: City University London Chat With Us; this should direct you to the Unibuddy widget to sign up.
Day-of Sign Up
You can also wait until the day of the live event to sign up via the event link. A page will be displayed via the link to either Sign-Up or Login.