Adding a video file to an email is not possible as emails do not support embedded video files. Instead, you may embed a thumbnail image of the video and link the video source to the image.
How To Insert A Video Thumbnail
We recommend using an image that has a play button over the top, this way people recognize it is a video that can be clicked on to be played. You can use tools like Canva.com or, just take a screenshot of the video playing on YouTube.
Next, click and drag the image styler to the section of your email template.
Place the thumbnail image you would like to use.
Finally, add your link to the video source on the image itself using the right sidebar while the image is highlighted/selected.