So you need to copy & paste it?
If you hold your left mouse button down and drag it across the website address, it should turn blue. This is called "selecting" the text. Once you've done that, right click. A little menu will show up with some options. With your left mouse button, click on "copy".
Reply on the board as you normally would, and when you want to insert the address, right click again. This time, left click on "paste" and that will copy the text to the reply.
Then just click on "post reply" like you normally would, and you should be in luck.
Sorry if that's not what you need to do, I'm half asleep today and could be misreading it.