private static string AccessToken;
public static string Free2ConveyUrl = "http://sandbox-api.free2convey.co.uk";

public static async Task Login(string username, string password)
{
    string result = null;
    string formContent = String.Format("grant_type=password&username={0}&password={1}", username, password);
    using (var client = new HttpClient())
    {
        HttpContent content = new StringContent(formContent, Encoding.UTF8, "application/x-www-form-urlencoded");
        client.DefaultRequestHeaders.Accept.Clear();
        var url = Free2ConveyUrl + "/token";
        HttpResponseMessage response = await client.PostAsync(url, content);
        if (response.IsSuccessStatusCode)
        {
            result = await response.Content.ReadAsStringAsync();
            result = result.Split(':', ',')[1].Trim('\"');
            AccessToken = result;
         }
     }
}