You can also highlight the word and click the little button on the toolbar that looks like Earth with an infinity symbol(it's supposed be a "link" in a chain) and it will ask you to input your url. Then it does all the hard work for you.
Under the quick reply there is a permissions box.
See where it says BB Code? Click that. Most, if not all codes are there on how to make spoilers, link URLS, ect.