Okay that is easy.

Use the "http://" button when responding or typing a new thread. (This button shows up right above your text box in which you type in).

After you hit the button, the first thing says enter text, basically that is whatever you want to name your link.

After you do that just click "ok" then it will tell you to enter the full url, after you do that just click "ok" again and you are done.