click the globe button on the tool bar above where you type text for posts after you highlight a selection of text. then go to your webpage on another tab, and put the address in the text box.(the button is right next to the globe with a red X, if you hold your cursor over the correct one it will say LINK)
an example is my signature at the bottom of my post it goes to my journal, you can change you signature in the "MY ROLLITUP" section, top right of your screen.