var username = "user";
        var password = "pass";
        var domain = "domain";

        var handler = new HttpClientHandler
        {
            Credentials = new NetworkCredential(username, password, domain)
        };

        var client = new HttpClient(handler);

        HttpResponseMessage content = client.PostAsync("http://mysite/_api/contextinfo", null).Result;

private async void BtnRestApi_OnClicked(object sender, EventArgs e)
    {
         var username = "user";
         var password = "pass";
         var domain = "domain";

         var handler = new HttpClientHandler
         {
             Credentials = new NetworkCredential(username, password, domain)
         };

          var client = new HttpClient(handler);

         HttpResponseMessage content = client.PostAsync("http://mysite/_api/contextinfo", null).Result;
    }