Unable to Consume Web Api in Xamarin

var keyValues = new List>

{

new KeyValuePair("username", UserName),

new KeyValuePair("password", Password),

new KeyValuePair("grant_type", "password")

};

var request = new HttpRequestMessage(HttpMethod.Post, "http://softstacksolutions.in/Token");

request.Content = new FormUrlEncodedContent(keyValues);

var client = new HttpClient();

var response = await client.SendAsync(request);

var jwt = await response.Content.ReadAsStringAsync();

JObject jwtynamic = JsonConvert.DeserializeObject(jwt);

var accessToken = jwtynamic.Value("access_token");

return accessToken;

this code works fine on simulator but faild but app crashes while this code is called

why?