void CreateCustomField()
        {
            List list = new List(listID);

            try
            {
                string newCustomFieldKey = list.CreateCustomField("NewCustomField", CustomFieldDataType.Text, null);
                Console.WriteLine(newCustomFieldKey);
            }
            catch (CreatesendException ex)
            {
                ErrorResult error = (ErrorResult)ex.Data["ErrorResult"];
                Console.WriteLine(error.Code);
                Console.WriteLine(error.Message);
            }
            catch (Exception ex)
            {
                //handle some other failure
            }
        }
        void CreateMultiOptionCustomField()
        {
            List list = new List(listID);

            try
            {
                List<string> options = new List<string>() { "Option 1", "Option 2", "Option 3" };
                string newCustomFieldKey = list.CreateCustomField("NewCustomField", CustomFieldDataType.MultiSelectOne, options);
                Console.WriteLine(newCustomFieldKey);
            }
            catch (CreatesendException ex)
            {
                ErrorResult error = (ErrorResult)ex.Data["ErrorResult"];
                Console.WriteLine(error.Code);
                Console.WriteLine(error.Message);
            }
            catch (Exception ex)
            {
                //handle some other failure
            }
        }