You cannot embed a script object (e.g. a video embed code) directly into a mass email. Scripts like this are considered security threats and are blocked by ISP's. So doing it would result in a near-zero delivery rate.
An alternative approach is to insert an image that represents your video. You would then insert that static image into your email. That image links to the actual video on YouTube, Vimeo, your website, or wherever you have it located.