Modified version of Scheduling Live Broadcasts from Google Sheets - with setThumbnail https://mashe.hawksey.info/2017/09/identity-crisis-using-the-youtube-api-with-google-apps-script-and-scheduling-live-broadcasts-from-google-sheets/

yt.gs

/**
 * @OnlyCurrentDoc
 */

YouTube.setTokenService(function(){ return getYouTubeService().getAccessToken(); });

// Read data from current sheet and create live events returning details back to sheet
function schedule_events(){
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("YT");
  new cUseful.Fiddler(sheet)
  .mapRows (function (row) {
    if (row.start_time && !row.youtube_url && !row.stream_key){ // if not already scheduled
      // format datetime to ISO 8601 format as per docs https://developers.google.com/youtube/v3/live/docs/liveBroadcasts#snippet.scheduledStartTime
      row.start_time = Utilities.formatDate(row.start_time, Session.getScriptTimeZone(), "YYYY-MM-dd'T'HH:mm:ss")+"+0100";
      
      row.end_time = Utilities.formatDate(row.end_time, Session.getScriptTimeZone(), "YYYY-MM-dd'T'HH:mm:ss")+"+0100";
      // step 1 - create a broadcast
      var broadcast_resource = insert_broadcast(row);
      // step 2 - create a stream
     // var stream_resource = insert_stream(row);
      // step 3 - bind broadcast to stream
      var bind_result = bind_broadcast(broadcast_resource.id, "STREAM_RESOURCE_ID");
      
      // step 4 - prep data to add back to the sheet
      row.youtube_url = "https://www.youtube.com/watch?v="+broadcast_resource.id;
      row.vid = broadcast_resource.id;
      setThumbnail(row.vid);
      
      
      var ingestionInfo = {"streamName": "STREAM_NAME",
                           "ingestionAddress": "rtmp://a.rtmp.youtube.com/live2",
                           "backupIngestionAddress": "rtmp://b.rtmp.youtube.com/live2?backup=1"
                          };
      row.stream_key = ingestionInfo.streamName;
      row.ingestion_address = ingestionInfo.ingestionAddress;
      
      row.start_time = new Date(row.start_time);
      row.end_time = new Date(row.end_time);
    }
    return row;
  }).dumpValues();
}

// The following is based on 
// https://developers.google.com/youtube/v3/live/code_samples/python#create_a_broadcast_and_stream
// LICENCE Copyright Google http://www.apache.org/licenses/LICENSE-2.0

// Create a liveBroadcast resource and set its title, scheduled start time,
// scheduled end time, and privacy status.
function insert_broadcast(options){
  var insert_broadcast_response = YouTube.liveBroadcastsInsert("snippet,status",
                                                               {"snippet": {
                                                                 "title": options.title,
                                                                 "description": options.desc,
                                                                 "scheduledStartTime": options.start_time,
                                                                 "scheduledEndTime": options.end_time,
                                                               },
                                                                "status": {
                                                                  "privacyStatus": options.privacy_status,
                                                                  "selfDeclaredMadeForKids": false
                                                                }
                                                               },{});

  Logger.log(insert_broadcast_response);
  return insert_broadcast_response;
}

function getLiveReusableStream(){
  var stream = YouTube.liveStreamsList("id,snippet,cdn", {mine:true});
  Logger.log(stream);
}

function setThumbnail(video_id){
  return YouTube.thumbnailsSet(video_id, {payload: DriveApp.getFileById("FILE_ID").getBlob()}) 
}

// Create a liveStream resource and set its title, format, and ingestion type.
// This resource describes the content that you are transmitting to YouTube.
function insert_stream(options){
  var insert_stream_response = YouTube.liveStreamsInsert("snippet,cdn",
                                                         {"snippet": {
                                                           "title":options.title,
                                                           "description": options.description,
                                                         },
                                                          "cdn": {
                                                            "ingestionType":"rtmp"
                                                          }
                                                         },{});

  Logger.log(insert_stream_response);
  return insert_stream_response;
}

// Bind the broadcast to the video stream. By doing so, you link the video that
// you will transmit to YouTube to the broadcast that the video is for.
function bind_broadcast(broadcast_id, stream_id){
  var bind_broadcast_response = YouTube.liveBroadcastsBind(broadcast_id, "id,contentDetails", {"streamId":stream_id});
  Logger.log(bind_broadcast_response);
  return bind_broadcast_response;
}