If your school's onMessage site is not yet live, the event registration will redirect to the login page. In order to resolve this, you need to add &pe=1 to the end of the registration URL.

So an event registration URL that looks like this:
https://[yourschoolname].myschoolapp.com/podium/default.aspx?t=XXXXX&rid=XXXXX

Would become this:
https://[yourschoolname].myschoolapp.com/podium/default.aspx?t=XXXXX&rid=XXXXX&pe=1