WebProxy class in Portable Class Library

Question

I'm building an Portable Class Library project. I seem not support WebProxy ( I'm preferenced the System.Net )

 

cookieContainer = new CookieContainer();
handler = new HttpClientHandler
{
CookieContainer = cookieContainer,
UseCookies = true,
UseDefaultCredentials = false,
UseProxy = true,
Proxy = new WebProxy("http://127.0.0.1:8888"), // It raised the error here: Error 1 The type or namespace name 'WebProxy' could not be found (are you missing a using directive or an assembly reference?)
}; 

 How to slove this ?