When you are on the page you want to link to, highlight the address in the window at the top (this one is http://www.usingenglish.com/forum/support-area/144199-creating-link.html) then either right-click and choose copy or press the Control key and C at the same time to copy it.
You can either paste it directly in, as I have above, or you can highlight a work, and then choose the little symbol that looks like a globe with a chain link under it, and paste the address into the box that opens up. I did that here.
You will want to have both windows open, so you can go back and copy it again if you do something like "cut" text in your post. The cut text will replace whatever you've copied.