Final answer:
To add a thumbnail URL on WordPress so that videos on pages can be indexed and served as video, you can embed the video with a featured image.
Step-by-step explanation:
To add a thumbnail URL on WordPress so that videos on pages can be indexed and served as video, you can embed the video with a featured image. To do this, go to your WordPress editor and click on the 'Add Media' button. Upload your video file and set a featured image for the video. This image will be used as the thumbnail. When you publish the post, the video will be indexed and served as a video.