I have no time to write example code, especially since this involves a fair few steps, but you'd need to create a separate css div layer under the passage z index, give it the appropriate size, and set a youtube video player to work on that layer, set the video player to autoplay when triggered, and then make the text passage transparent so the video layer shows through.
You'll need to use pure javascript and html to do this. It's not a feature of Harlowe, or any story format for that matter.
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You'll need to use pure javascript and html to do this. It's not a feature of Harlowe, or any story format for that matter.