Press URL.

Type in the URL.

Then after the URL is put in, part of the BB code will be highlighted. It will look the same as the URL, but if you change it, it will mask the code.

HTML Code:
[URL="www.onrpg.com"]www.onrpg.com[/URL]
See the right most www.onrpg.com? The one without quotation marks is the one that will appear, whereas the one with quotation marks will be the address it will lead to.

So...
HTML Code:
[URL="www.onrpg.com"]buttfaces[/URL]
Will become:

buttfaces