Across the top of the text box when you are posting, there is a line of icons. One of them is a chain (to symbolize a link), but it will be grayed out until you do this: Type or paste in the text you want to make live, then highlight it. The link icon will become active. Click on it and the rest is self explanatory. The center field allows you to specify that your link will open in a new window; that's usually the best option but use your own judgement.