easiest way to add the "post to Hacker News" button on your website
Perhaps, but I question the value of such buttons. With my data set of 1, I can tell you that I personally find them intrusive and ugly. If I want to submit a page to HN or some other aggregator, it will get submitted irrespective of a little button's existence.
When I really like an article / post, I will try to "share the love" with the social media vehicle that website recommends. Having just one button for one network / aggregator tells a lot.