For comments you have to just type the url, no text or tags.<p><a href="http://google.com/" rel="nofollow">http://google.com/</a><p>For submissions you can submit a url <i>or</i> text but not both, and URLs in the text don't get parsed.
What you could do is submit a post describing the website and
why HN readers should be interested in it. Then immediately post a comment with a clickable link to the URL.