To post a link go to the boxes right above the post and there is an icon that looks like a ball with an oval just below it, select it and you'll get the link helper box.

You can also highlight the link and hit Ctrl+C to copy to your computer's buffer, then come here to your post and put the cursor where you want to insert the link or text and hit Ctrl+V.

Last option is to just type in the web page manually.