Unfortunately, adding video content to a template and having it play in-line in the sent message is not available at this time.
However, you can insert an inline image (or gif!) in your templates by clicking on the 'Insert Image' button (with the mountain icon) in the edit tools above the text field. One suggestion would be to insert a still image of the intended video, then making that image clickable by highlighting it and clicking the hyperlink icon to input the video URL.
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