You can add a YouTube video in a discussion reply by adding the URL of the video as a link to the response.
Type or copy/paste the YouTube URL in the text box . By default, a small embedded image of the video will appear, showing the video you have selected. The link will also include a small thumbnail with an embedded video. Add alt text for the video in the Alt text for inline preview field . If you do not want to include the embedded video, click the Disable inline previews for this link checkbox . When you are ready to insert the video link into the Rich Content Editor, click the Insert Link button .
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The embedded media can be viewed inside the discussion post  or by clicking Link  to view the media in a new window.