Another way to do this is to ensure that you have already selected and copied your URL.
Then, in the post that you are writing, type the text that you would like like to be clickable text.
Having done that, now highlight that text with your cursor and click the little icon at the top of this posting box that looks like a the earth with a little chain link underneath.
This will bring up a new box that lets you insert a URL - paste the URL that you selected earlier into this box and click OK.
When you go back to the post that you are writing, you will see that your highlighted text has now changed colour and become a clickable link.
Hope I haven't confused you