When you are typing your message in the post reply message box look right above and you will see two rows of icons. The icon in the second row that looks like a blue and green world with a chain link in front of it can help you post a link. (This icon is directly below the white smiley face that is in the first row.) When you click this icon a dialog box opens and offers a field in which you can enter the URL to where you want to link. Type or paste your URL into the dialog box. Make sure you don't end up with two strings of - http:// - if you are pasting a copied URL.)
You can display your link two ways. You can display the full text of your URL so it appears "as is" in the body of your message or you can mask your URL so the actual URL is hidden and word(s) or picture(s) serve as the link. (for example click here)
To mask your link highlight the text you want to represent the link and then use the same blue globe icon mentioned above.
If you have something highlighted before you click the globe you will mask the URL if nothing is highlighted when you click the globe you will see the text for the whole URL.