You can add a YouTube video in a Discussion reply by adding the URL of the video as a link to the response.
In Course Navigation, click the Discussions link.
Click the discussion title.
Create a new discussion entry by clicking the Reply text field.
Embed a URL link in the discussion reply by clicking the Insert/Edit media content link.
Paste the URL in the Source text box and click the Insert Link button.
Note: You can also automatically insert a YouTube video by copying and pasting the link directly into the Rich Content Editor. However, it is better to create an accessible hyperlink by typing the YouTube video title in the Rich Content Editor, highlighting the title, and then inserting the YouTube link.
Click the Post Reply button to post your discussion reply.