There is an error in the API docs here: https://developers.cloudflare.com/stream/video-playback/player-api/
The Methods section has text from the above section.
A URL for an image to be shown before the video is started or while the video is downloading. If this attribute isn’t specified, a thumbnail image of the video is shown.
The video id from the video you’ve uploaded to Cloudflare Stream should be included here.