You can do this manually (personally it's faster when I'm typing so I don't have to switch to the mouse):
(a href="URL HERE")LINK NAME HERE(/a)
- replace () with <>
Result:
Pokemon DB
Or, you can click the little world w/ an arrow above the text box:
Which will open a pop up box for you to insert the URL.
Then a set of text will appear:
(enter link description here)(1) <---- obviously replace this w/ whatever you want.
(1): http://pokemondb.net/ <---- this part will appear at the bottom of your text box and will not be visible in your answer.
Result:
enter link description here
Edit from m00:
You can also hit Ctrl and L, and then paste the link in the box. The result would be the same as if you hitting the button above.