You can do this here:

https://leanpub.com/YOUR_BOOK/settings/web_page_media

...making sure to replace YOUR_BOOK with your book's unique "slug".

You can also find this page if you go to the Overview page for your book and select "Video" under "Settings":

On that page, you will see an option to enter the URL for your video:

Be sure to enter only the part of the video's URL that comes after the /. For example, you should do this:

...but you should not do this:

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