Pull up the old thread. Copy the address on your bar to your clipboard. To do this, Highlight it and right click then go to copy, or highlight it and hold down cntrl and press "C"
Paste it in your reply by right clicking and going to paste or hold down cntrl and press "V"
to take it a step further, you can RIGHT click on a post # (top right of each post) and click copy and when posting hit control V to paste it in your message. That will take the link to only that reply.