The countdown will count down to the next event that is categorized by "Live Service". When you are creating the event, set the date and time, and be sure to add the category "Live Service"

When the event begins the countdown timer will switch to say "Live Now" with a "Watch" button.

You can update where the "Watch" button links to by going to Content > Link Lists > Countdown bar and editing the link and what the button says there.