Open the website you want to copy in a second tab. Left click your mouse after words in the address bar and drag your mouse all the way to the left/beginning of the site address until it's all highlighted then right click, select 'Copy'. Go to your thread reply box, left click your mouse in the box so that you get a cursor, press right click and select 'Paste'. Once it's there, you can type words in front or behind the copied link string but don't type within it. Note that it won't look like a link (blue type) until you submit the reply.