Ask HN: Why are URLs clickable in comments but not in main thread?
I'm sure this can be changed easily. But it is not. So I guess there's a reason behind it. Also, it makes no sense for authors to post their own links in the comments.
You can't. (This is to prevent people from using this
method as a way of submitting a link, but with their
comments in a privileged position at the top of the
page. If you want to submit a link with comments, just
submit it, then add a regular comment.)