For S-O question: This method returns 204.

Client.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net.Http;
using System.Net.Http.Headers;

namespace Client
{
class Program
    {
        static void Main(string[] args)
        {
            Task.WaitAll(Go()); // block while the task completes
        }

        static async Task<HttpResponseMessage> Go()
        {
            string serviceUrl = "http://localhost:4097/api/test/KillPerson";
            var httpClient = new HttpClient { BaseAddress = new Uri("http://localhost:4097") };
            httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

            var httpContent = new StringContent("{a: 'b'}", Encoding.UTF8, "application/json");
            var response = await httpClient.PostAsync(serviceUrl, httpContent).ConfigureAwait(false);
            response.EnsureSuccessStatusCode();
            return response;
        }
    }

Service.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;

namespace WebAPITest.Areas.Test
{
    public class TestController : ApiController
    {
        // POST api/<controller>
        [ActionName("KillPerson")]
        [HttpPost]
        public void Post([FromBody]string value)
        {
        }

    }
}