1. Highlight the text you wush to include in your link.
  2. Click the "Link" button in the toolbar above the message box. (It's the button that looks like a globe with a piece of chain under it.)
  3. In the box that pops up, it will ask you for a prompt to include text with your link. If you've already highlighted text, the text will automatically appear in the box.
  4. Press enter.
  5. In the following box, type in or paste the URL of your link.
  6. Press enter.


More information may be found here.