Example of Episerver Forms 3.0 and 4.0 Webhook handling.

WebApiRouting.cs

using EPiServer.Framework;
using EPiServer.Framework.Initialization;
using System.Web.Http;

namespace AlloyTen.Business.Initialization
{
    [ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
    public class WebApiRouting : IInitializableModule
    {
        public void Initialize(InitializationEngine context)
        {
            // Pass a delegate to the Configure method.
            System.Web.Http.GlobalConfiguration.Configure(Register);
        }

        public void Uninitialize(InitializationEngine context)
        {

        }

        public static void Register(HttpConfiguration config)
        {
            //Force JSON responses on all requests            
            config.Formatters.Clear();
            config.Formatters.Add(new System.Net.Http.Formatting.JsonMediaTypeFormatter());            
            config.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;

            // Attribute routing.
            config.MapHttpAttributeRoutes();

            // Convention-based routing.
            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );
        }
    }
}

WebhookDataController.cs

using EPiServer.Logging.Compatibility;
using Newtonsoft.Json.Linq;
using System;
using System.Web.Http;

namespace AlloyTen.Controllers.WebApi
{
    [RoutePrefix("api/webhook")]
    public class WebhookDataController : ApiController
    {
        private static readonly ILog _Logger = LogManager.GetLogger(typeof(WebhookDataController));

        [Route("contactus")]
        [HttpPost]
        public string ContactUs([FromBody]JToken jsonbody)
        {
            try
            {
                string name = jsonbody["Name"]?.ToString();
                string comment = jsonbody["Comment"]?.ToString();
                string submitDate = jsonbody["SYSTEMCOLUMN_SubmitTime"]?.ToString(); 

                _Logger.Debug($"Contact us form submitted with name = {name}, comments = {comment} on {submitDate}!");
            }
            catch (Exception e)
            {
                _Logger.Error("Failed to process contact us!", e);
            }

            return string.Empty;
        }
    }
}