Open the page where you watch the desired youtube video.
Make sure your reply settings automatically parse links in text, then copy(Ctrl+C) the link from the watch page(it will say "watch" somewhere in the address) and paste(Ctrl+V) the link into the reply box.
The result will look like this: https://www.youtube.com/watch?v=WCdJ2Bc3FrQ&list=PLQ7BMI4rMWmfkIWViz01wGHAturaPZ5Cx&index=7&t=0s