|Sorry, you can't.|
But it *is* possible to do some (highly) limited kinds of HTML-like things using just the "Natural text" formatting. For example, to include a hyperlink, do it like this: <http://chips.kaseorg.com/>. You just surround the URL with angle brackets during editing, and it'll come out as a hyperlink with the angle brackets removed. (The reason why you see the angle brackets here is because I typed in <<http://chips.kaseorg.com/>>)
For more information, or if you wonder why HTML is banned here, please check out http://faqomatic.sourceforge.net/fom-serve/cache/428.html.