First login to your promoter account, and then find the event you wish to amend.
Do this by going to View All, then selecting the event in the list that appears.
On your selected event, select ‘Edit event details’
To set a countdown timer, select ‘Start ticket sales on a certain date?’
The date that you set, is when the countdown timer will end, and reveal your tickets.
Once setting the on sale date, make sure you check the tickets you wish to go live, are set as Available.