Returning responses from Web Api

My web api needs to return a response. Can't seem to get it right. Here is the code for the controller.

//Controller Code
        // POST api/<controller>
        [HttpPost]
        //public IActionResult Post([FromBody] customersT customers)
        public string Post([FromBody] customersT customers)
        {
            SageHelper sageHelper;

            if(!ModelState.IsValid)
            {
                return "Invalid XML"; //BadRequest(ModelState);
            }
            else
            {
                sageHelper = new SageHelper(customers);
            }


            return sageHelper.ReturnValue; //Ok();
        }

Here is the helper class that will do the post to Sage

        public static async Task CreateCustomer(string uri, customersT customers)
        {
            var customer = new
            {
                CustomerNumber = customers.Items[0].custNo,
                ShortName = customers.Items[0].custNo,
                CustomerName = customers.Items[0].custAddresses[0].name,
                GroupCode = "RTL"
            };

            await SendRequest(new HttpMethod("POST"), uri + @"AR/ARCustomers", customer);
        }

        public static async Task<object> SendRequest(HttpMethod method, string requestUri, object payload = null)
        {
            HttpContent content = null;

            string responsePayload = "";
            // Serialize the payload if one is present
            if (payload != null)
            {
                var payloadString = JsonConvert.SerializeObject(payload);
                content = new StringContent(payloadString, Encoding.UTF8, "application/json");
            }

            // Create the Web API client with the appropriate authentication
            using (var httpClientHandler = new HttpClientHandler { Credentials = new NetworkCredential("USER", "PASSWORD") })
            using (var httpClient = new HttpClient(httpClientHandler))
            {

                // Create the Web API request
                var request = new HttpRequestMessage(method, requestUri)
                {
                    Content = content
                };

                // Send the Web API request
                try
                {
                    var response = await httpClient.SendAsync(request);
                    responsePayload = await response.Content.ReadAsStringAsync();

                    var statusNumber = (int)response.StatusCode;
response.StatusCode);

                    if (statusNumber < 200 || statusNumber >= 300)
                    {
                        // Console.WriteLine(responsePayload);
                        throw new ApplicationException(statusNumber.ToString());
                    }
                }
                catch (Exception e)
                {
                    Environment.Exit(0);
                }
            }

            return string.IsNullOrWhiteSpace(responsePayload) ? null : JsonConvert.DeserializeObject(responsePayload);
        }

When the HTTP post hits the controller, it sends the request off to the SageHelper object which processes the data and pushes into Sage. The SendRequest method returns a string which I actually don't want for this project. Where it gets the response.statusCode from the response object, I am trying to get that set into a public property of the SageHelper class

public string ResponseCode { get; set; }

Normally, I would just put ResponseCode = response.statusCode; in the SendRequest method but I can't see the property from that method.