Install youtube api for your application first:
PM> Install-Package Google.Apis.YouTube.v3
Add the following code in your cs file:
using Google.Apis.Auth.OAuth2;
using Google.Apis.Services;
using Google.Apis.Upload;
using Google.Apis.Util.Store;
using Google.Apis.YouTube.v3;
using Google.Apis.YouTube.v3.Data;
Further details:
https://developers.google.com/api-client-library/dotnet/guide/aaa_oauth