This is an embed:

So is this, with only three spaces of indentation:

This is not, because it's fully indented:

https://www.youtube.com/watch?v=dQw4w9WgXcQ

This is not, because it has extra bits after the URL:

https://www.youtube.com/watch?v=dQw4w9WgXcQ <-- you gotta watch this!

This is not, because it's in a fenced code block:

https://www.youtube.com/watch?v=dQw4w9WgXcQ

And this isn't either because it's inline:

Embeds can't be nested in other blocks:

• https://www.youtube.com/watch?v=dQw4w9WgXcQ
  • https://www.youtube.com/watch?v=dQw4w9WgXcQ

This isn't valid because it's a lazy paragraph continuation: https://www.youtube.com/watch?v=dQw4w9WgXcQ

^ This is fine, though