Unfortunately you need to enter any of the worlds (doesn’t need to be the same one as the event is scheduled for).
You’ll then get an pop-up inviting you to to join the event and you’ll be transported to the starting pen for the event (or decline if you’d like to do other riding first - there will be a button in the lower-left section of of the screen with a countdown timer to the start of the event; along with regular pop ups that ask if you would like to join the event now).