I don’t see a way. It doesn’t mean there isn’t, though.
But if you use roundcube.dreamhost.com, you can log into your webmail account. Just type your entire email address in the username field, even though it says @dreamhost.com after the field.
When you compose in Roundcube, there’s a radio button at the bottom that lets you compose in HTML or Plain Text. With HTML mode, you get a toolbar that lets you create a link (chain icon) for highlighted text.