I was going to explain how to do it, but then I just noticed something...
If you look at the second line above the "reply" window, you'll see ALL the way on the right a mouse cursor inside brackets. If you mouse-over that, it will say that it means "Toggle View".
It seems that toggles a view between a rich-text editor and a more HTML-style editor. If you go to the rich-text editor, and you highlight text, click the "Insert Hyperlink" button, it'll allow you to make that text into a link.
If you don't and you highlight text and hit that button, it will put a "url" in brackets at the beginning of your text and a "/url" in brackets at the end of your text. If you manually change that to "url=http://www.hyperlink.com" inside those brackets, it will make the text within the brackets link to whatever you put after the "url="...