For this example I will be using "A dart hit his head" by Squiggly, but you can use any YouTube video.
Simply move your cursor to the search bar and drag it over the part you want to copy. Don't forget to omit the last part of the URL (the part about the channel).
You have already found your video's link, now its time to choose the dimensions. Though Most videos have a 4:3 aspect ratio, not all do. The one I chose, for example, has a 2:1 aspect ratio.
When you are done, your code should look something like this:
Just remember to remove the things in curly braces and replace them with your own numbers/link