Extract access_token from response

using (var client = new HttpClient())
var baseAddress = new Uri("https://xxx.mmmm.com/token");

var form = new Dictionary<string, string>
{"grant_type", "password"},
{"client_id", "xxx"},
{"username", "x@uo.com"},
{"password", "xxxx"}

HttpResponseMessage response = new HttpResponseMessage();

using (response = await client.PostAsync(baseAddress, new FormUrlEncodedContent(form)))
var resultString = response.Content.ReadAsStringAsync();

I use the above code to make a POST request for getting access_token. How do I get the access_token value that is returned?