Ok, it's like this...
In the reply screen, under the 'underline' button, there is a button called 'insert Hyperlink' - it looks like a picture of the world.
Click on it, type (or paste, because you've already copied the URL from your web browser,) the URL in, then the next step directs you to type in the words to click on to direct the reader to the URL. Viola!!
Like so...