gistfile1.js

// this function handles the form submit
function addVideoToPlaylist(e){
  // 'Youtube url' is the question title from our form. If you change your question change this value
  // youtube_parser() takes the url and strips the video_id
  var id = youtube_parser(e.namedValues['Youtube url'][0]); 
  var desc = e.namedValues['Comment'][0]; // if you would like to add a note with playlist item add form field
  addToPlaylist(id, desc);
}

// This function is taken from the YouTube API doc (Javascript example) 
// https://developers.google.com/youtube/v3/docs/playlists/insert 
//
// Add a video to a playlist. The "startPos" and "endPos" values let you
// start and stop the video at specific times when the video is played as
// part of the playlist. However, these values are not set in this example.
function addToPlaylist(id, desc, startPos, endPos) {
  // this is my playlist id and you need to replace with your own
  // To get this in your YouTube channel create or go to you playlist 
  // and the list id will be in your browser address bar. Basically everything 
  // after https://www.youtube.com/playlist?list=
  
  var playlistId ="PLlMXkWgzMBtOZGp6beg10WqKb66wIk9eu";
  
  // the API requires a resource object. This next bit builds it
  var details = {
    videoId: id,
    kind: 'youtube#video'
  }
  if (startPos != undefined) {
    details['startAt'] = startPos;
  }
  if (endPos != undefined) {
    details['endAt'] = endPos;
  }
  if (desc == undefined) { // HT @mambelli
    var desc = "";
  }
  var part= 'snippet,contentDetails';
    var resource = {
      snippet: {
        playlistId: playlistId,
        resourceId: details
      },
      contentDetails:{
        note: desc
      }
    };
  // cheating slightly and doing no error handling. When you setup the form submit you can choose to recieve 
  // notification of errors. The form response will be submitted to the spreadsheet regardless.
  var request = YouTube.PlaylistItems.insert(resource, part);
}

// Little helper..
// http://stackoverflow.com/a/8260383/1027723
function youtube_parser(url){
    var regExp = /^.*((youtu.be\/)|(v\/)|(\/u\/\w\/)|(embed\/)|(watch\?))\??v?=?([^#\&\?]*).*/;
    var match = url.match(regExp);
    if (match&&match[7].length==11){
        return match[7];
    }else{
        throw("Url incorrecta");
    }
}