Within the tournament admin, go to "Events". From here you can add events to your tournament page.
The last step is setting up details such as which game is being played, what format is going to be used (1v1, 4v4 for example), and registration settings.
If you're having trouble adding the event because you're getting an error that says the event must be within the tournament start/end dates, you likely just need to adjust your tournament's Start Date and End Date to include the desired date for your event. You can do this by:
Adjusting the Start Date and End Date options to include the time you want
For more information on "Event Creation" please refer to this article.