Create a custom event provider

using EPiServer.Events;
using EPiServer.Events.Providers;
using EPiServer.ServiceLocation;
using System;
using System.Threading;
using System.Threading.Tasks;

namespace CustomEventProvider
  {
    [Options]
    public class CustomEventProviderOptions
    {
      public string CustomSetting { get; set; } = "custom setting";
    }
    
    public class CustomEventProvider : EventProvider
    {
      public CustomEventProvider(CustomEventProviderOptions options)
        {
        }
    
        public override Task InitializeAsync()
        {
          //Put initialization code here, example waiting for external callback and call OnMessageReceived(messages)
        }
    
        public override Task SendMessageAsync(EventMessage message, CancellationToken cancellationToken)
        {
          //Put code to sends the provided message to other related sites.
        }
        public override void SendMessage(EventMessage message)
        {
          throw new NotSupportedException("It doesn't need becuase there is already async version.");
        }
    }
  }

{
  "EPiServer" : {
    "Cms" : {
      "EventProviderOptions" : {
        "Provider" : "CustomEventProvider.CustomEventProvider, CustomAssembly"
      }
    },
    "CustomEventProviderOptions" : {
      "CustomSetting" : "custom setting"
    }
  }
}