Type your message, then go back and select (highlight) the text that you want to turn into a link. After you select the text, go to the top of the window and click on the little planet earth with the 2 link chain at the bottom of it. A window will open into which you paste the url that goes with the text. Click okay and your text is now a link.
Sample
This is how it looks behind the scenes but there are brackets on either end that make the coding disappear:
URL="https://www.mobileread.com/forums/showpost.php?p=209245&postcount=2"]Sample[/URL