How to add thumbnail upon upload

[ghost]

The class Google_Service_YouTube_VideoSnippet has a method setThumbnails that takes as a parameter an instance of Google_Service_YouTube_ThumbnailDetails which has two interesting methods called setDefault and setStandard which takes in parameter an instance of Google_Service_YouTube_Thumbnail which itself has a method called setUrl. This is the method I use to set the thumbnail url. I've also set the width and the height just in case, making sure the minimum width is 640.

However, upon upload the thumbnail I chose is not set. Why? What am I doing wrong?
( I want to know if it's possible to set the thumbnail before try this thumbnails/set and sending a second request.)

 $thumbnail_service = new \Google_Service_YouTube_Thumbnail();
    $thumbnail_details = new \Google_Service_YouTube_ThumbnailDetails();

    $thumbnail_service->setUrl($thumbnail_url);
    $thumbnail_details->setDefault($thumbnail_service);

    $snippet = new \Google_Service_YouTube_VideoSnippet();
    /*
      Other video info added to snippet
    */
    $snippet->setThumbnails($thumbnail_details);

StackOverflow: how-to-add-thumbnail-to-youtube-video-upon-insert