First, change your status to allow pop ups.
Second, on a second tab, pull up the website you're wanting to link to.
Now go back to your new message you're trying to post & click on the <b>http</b> button at the top of "message text" box
Copy/paste the link you want to post, enter "OK"
Then another line will appear with the word "text" in it, over-ride that with a description of your link, or often I just type in the word "LINK". Hit "OK"
The link should be there!