It’s part of the login subheader setting.
I think in the example something like this has been used:
Use this space to include some important information about the site before signing up, and even a video: <iframe width="560" height="315" src="https://www.youtube.com/embed/u7iW9DyFw7M?si=SQImwxaPkPVQgaNt" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
You can get the embedding code from the share option on YouTube.