If you have preordered a ticket to an online concert campaign you would be able to watch the show on the campaign page where you bought tickets. To find the concert go to the Pledges section on your Profile page.
Also, you will be notified in one hour before the show starts via the app notification and email with the link and access code (if you bought a few tickets and you need to share with your friends or if you want to watch the show on TV).