GetYouTubeService.cs

private async Task<YouTubeService> GetYouTubeService(string userEmail)
{
    UserCredential credential;
    using (var stream = new FileStream("client_secrets.json", FileMode.Open, FileAccess.Read))
    {
        credential = await GoogleWebAuthorizationBroker.AuthorizeAsync(
            GoogleClientSecrets.Load(stream).Secrets,
            new[]
            { 
                YouTubeService.Scope.Youtube,
                YouTubeService.Scope.Youtubepartner,
                YouTubeService.Scope.YoutubeUpload,
                YouTubeService.Scope.YoutubepartnerChannelAudit, 
                YouTubeService.Scope.YoutubeReadonly 
            },
            userEmail,
            CancellationToken.None,
            new FileDataStore(this.GetType().ToString()));
    }

    var youtubeService = new YouTubeService(new BaseClientService.Initializer()
    {
        HttpClientInitializer = credential,
        ApplicationName = this.GetType().ToString()
    });

    return youtubeService;
}