HTML code is not allowed by any means. Use BB code. You can even go to the advanced post windows and create a link from it's buttons. It's not that hard to do. The BB code for a URL is [url=http://website.com]http://website.com[/url], or if you want to name you're link, then it is [url=http://website.com]the_website_link_name[/url]. You can use spaces in the URL name, but not in it's address.
Check out this part of our FAQ to learn more (clicky).