To set the invoker as our custom invoker

MyOperationBehavior.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.ServiceModel.Description;
using System.ServiceModel.Dispatcher;
using System.Text;
using System.Threading.Tasks;

namespace TokenValidator
{
    public class MyOperationBehavior : IOperationBehavior
    {
        public void AddBindingParameters(OperationDescription operationDescription, System.ServiceModel.Channels.BindingParameterCollection bindingParameters)
        { }

        public void ApplyClientBehavior(OperationDescription operationDescription, System.ServiceModel.Dispatcher.ClientOperation clientOperation)
        { }

        public void ApplyDispatchBehavior(OperationDescription operationDescription, System.ServiceModel.Dispatcher.DispatchOperation dispatchOperation)
        {
            IOperationInvoker defaultInvoker = dispatchOperation.Invoker;
            dispatchOperation.Invoker = new CustomInvoker(defaultInvoker);
        }

        public void Validate(OperationDescription operationDescription)
        { }
    }
}