There are cases when you want to show the countdown right before the beginning of your pre-recorded live stream. In order to do so, take the following steps: 

1. Setup a Countdown layer

2. Add a layer with Video, which you're planning to stream after a countdown.

3. Hide a Video Layer and start streaming.

4. Once you're ready to switch from a Countdown to the pre-recorded video, hide or remove a Countdown Layer. Then unhide a Video Layer, and click "Sync with Live" button to make your changes visible to your audince.