【新手求解】关于SharePoint 2013 REST基本操作的问题

 HttpWebRequest endpointRequest =
  (HttpWebRequest)HttpWebRequest.Create(
  "http://<site url>/_api/web/lists");
endpointRequest.Method = "GET";
endpointRequest.Accept = "application/json;odata=verbose";
endpointRequest.Headers.Add("Authorization", 
  "Bearer " + accessToken);
HttpWebResponse endpointResponse =
  (HttpWebResponse)endpointRequest.GetResponse();

这个是微软官网上的示例代码 ，但是不知道accessToken怎么获得，说的不太清楚，有知道的吗 谢谢