To link to another post, simply type @ and the post's number. For example, if you'd like to link to post 47, type @47 in the body of your post. This way, users will be able to view post 47 by clicking "@47" in your post.
To find the number of a particular post, find the post in your Q&A feed in the left portion of the page. Place your mouse cursor on the post so that the inverted triangle appears. Then, place your mouse cursor over the triangle so that the dropdown menu appears. You will see the post number: