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 because there is already async version.");
    }
  }
}

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