When you click on the reply to topic link the next window to open will look like this.
On the third line you'll see the URL icon button. Click on that button and you'll get text such as is in this pic.
Then you need to insert the address and descriptive text.
Note that the address after the = is in quotations before the bracket, ].
You can see how I did this in my signature when you click on the Quote icon/button which is next to last on the line with the URL icon/button.