using System.Net;

WebClient wc = new WebClient();

string strUserAgent = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/53.0.2785.143 Safari/537.36";

string strReferer = "https://www.google.com.tw";

wc.Headers.Add("user-agent", strUserAgent);
wc.Headers.Add("referer", strReferer);