Add the source parameter to the url as described in the google api documentation: YouTube API API Link for iframe attachments
As an additional security measure, you should also specify the origin parameter in the URL, specifying the URL scheme (http: // or https: //) and the full domain of your main page as the parameter value. While the source is optional, it also protects against malicious third-party JavaScript that enters your page and takes control of your YouTube player.
<iframe id="player" type="text/html" width="640" height="390" src="http://www.youtube.com/embed/u1zgFlCw8Aw?enablejsapi=1&origin=http://example.com" frameborder="0"></iframe>
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