On YouTube, below the video click the "share" button. And then click "embed". Copy the code and paste it here for the video to appear. No tags are needed, just the code. If you only want the link and not the video to appear here, just click the link code that first appears when you click on "share". Copy it and paste here. After that, highlight it and click on "URL" that appears above when making a post. Most other video sites have something similar to this.
It is better to embed the video here, because more people will see it than if they have to click a link.
The part that is highlighted is what you would copy to paste here.