When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Also you can start another tab in your browser and go to the post you are trying to link. Then just copy the address in your address bar in your browser by a right click on the address bar and select copy, then click on the post you are doing to post in your first tab & paste it in. Sounds long, but just takes a few seconds really..
If you want a specific post in a thread, look at the numbers top right of each post:
"permalink"
like this one, for the post right above this one. You can either use the "add link" button or just paste the link into the page and the forum will automagically work on it.
E.g. Pasted and then using the "add link" button - see, same result.