Member Rara Avis
If you just type it in, exactly as you see it in your browser's Address (or Location) bar, that will work.
Better, use copy and paste. Click in the Address (or Location) bar, and that should highlight the whole URL. Hold down the Control key (CTRL), then press C (for copy). Click back into your post or reply window, hold down the Control key again, and then press V (for, uh, paste?). If you aren't comfortable with CTRL shortcuts, you can also copy and paste using the right-click menu on Windows.
Any way you can get the URL into your message, the system will automatically recognize it and convert it into a clickable link.