Here are the steps you need to take:
Click on "Switch to Full Reply Screen" at the bottom of the box where you type your post.
Click on the 2nd icon at the top of the text box. It looks like an earth with a gray chain link in front of it.
Copy the link you wish to share and paste it in the small box that pops up....Then click "ok"
Put something to Identify the link you are sharing such as "click here", or "cute dog", etc in the next small box that pops up and click "ok"
Your link will now be in the white text box where you type your posts. You can add more information or just hit "submit."