Above the text box you type in is a mostly green button with a + sign, in green on it. Click that and a box will come up, paste your link into it. Click OK. Another box will come up and you can give your link a name by typing in the box. Click OK again and the HTML will show in your reply box.
Just continue typing after that if you wish and submit the post. The name you called your link will show in the post as an underlined link.