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Enter your comment in the comment box and and click "Post" to submit. The comment will be attached to a time code (e.g. 00:09). Your comment will appear in the discussion panel in order of its time code.
You can make keywords in your comment searchable by adding a hashtag (#) in front of it. Doing this also makes your comment stand out a little more and gives people an idea of what your comment is about before they actually read it.