Flex includes a large Featured Image area, and since version 1.1, Flex has also supported video in this location. You can enable video by adding a video embed link to the field provided on the post/page editing screen (but you will still need to add a Featured Image, too):
Videos provided by YouTube and Vimeo, for example, have a specific format for their embed URLs:
Other providers will also have an embed URL that is not the same as the Link URL you use for sharing a video.
Once a video embed URL is provided, a play button will appear in the Featured Image area on the Page or Post.
Note: a Featured Image is required in order for the Video to appear. This image will also act as a placeholder, so we encourage you to use a snapshot from the video or else use an image that is relevant to the video.