When publishing a collection, you may want to include a trailer to encourage potential customers to subscribe or purchase the collection. For collections with trailers, the watch trailer button will appear for all potential customers as well as signed in customers and subscribers. The trailer video does not need to published in order for it to play once assigned to a collection. In fact, we recommend you don't publish that trailer video to avoid the trailer becoming a standalone video on your catalog page.
Navigate to the collection you want to add a trailer for under Content > Collections and click Edit. Under the Appearance tab click Choose Video in the Trailer Video section.
The Watch Trailer button will appear on the video page under the video player.