In general, we expect posts to be self contained so that all details needed are provided & users do not have to go off-site to find out what you're really questioning. Hence, you should provide a link for additional reference to the post, but it should not be a requirement to understand what it is you are looking at and/or questioning.
Ben's comment ...
Questions and answers are followed by several links, one of which is "share." Clicking "share" opens a little modal window with the permalink in a text box and a button that lets you copy it to your clipboard. But you can also right-click (or long-tap) the "share" link and copy it to the clipboard using the context menu in your browser, same as other links.
Besides the points made in the answers by David Z & Emilio Pisanty, make sure to mention in the question
lecturer, title, etc, of video, so the link can be recreated in case of link rot.
minute in video, so the reader doesn't have to watch the entire video.