Allow up to 3 hours for ads to start displaying correctly before contacting Google DFP customer support. If it has been longer than 3 hours, you can reach Google support here: https://support.google.com/dfp_premium/?hl=en#topic=28132&contact=1
You can enter a video preroll URL in the designated field. The video preroll ad will automatically play when the player starts or when a user taps play on a mobile device. If supported, the video preroll can also serve companion ads such as big box, leaderboard, or mobile banner ads.