The workout can be started by clicking on the white triangle (Play button) in the center of the screen. If the settings are configured for Auto-Pause, the workout will start automatically when you start pedalling. Pausing a workout can be done manually by clicking the pause button on the screen while the video is playing. The Pause button will become visible when the cursor is moved.
The space bar on your keyboard is a shortcut key for this function. (Mac and Windows)