senderAuthenticationTokens call returns error code 415 Unsupported Media Type

string jsonString = "{\"packageId\":\"" + packageId.Id + "\"}";
string url = "https://sandbox.esignlive.com/api/senderAuthenticationTokens";
HttpClient myClient = new HttpClient();
myClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", apiKey);
myClient.DefaultRequestHeaders.Add("Accept", "application/json");
StringContent jsonContent = new StringContent(jsonString, Encoding.UTF8, "application/json");
var response = myClient.PostAsync(url,jsonContent).Result;
Debug.WriteLine(response.Content.ReadAsStringAsync().Result);

using (var client = new WebClient())
 {
        client.Headers["Content-Type"] = "application/json";
        client.Headers["Accept"] = "application/json";
        client.Headers["Authorization"] = "Basic " + apiKey;
        string response = client.UploadString("https://sandbox.esignlive.com/api/senderAuthenticationTokens", "{\"packageId\":\"" + packageId.Id + "\"}");
                    
        Debug.WriteLine(response);
}