To continue with a post, or refer back to it you would need to copy the url (address bar) to your new post.

At your old post in the old forum, click on the address bar, (shown highlighted in the image below) then hold down Ctrl key and hit the C key, this will copy the address for you.

Then come to your new post here, fill in your heading, then in the text box hold down the Ctrl key and hit the V key.  This will paste your link.