ExtendCommandWithData.cs

public class ExtendCommandWithData<TRequest, TResponse> :  IPipelineBehavior<TRequest, TResponse>
{
    public async Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
    {
        // do something with request to add the data

        //if (typeof(TRequest).IsAssignableFrom(typeof(ICommand)))
        //{
        //    request.AddUserId("id");
        //}

        var response = await next();      


        return response;
    }
}

ICommand.cs

public interface ICommand
{
    string UserId { get; set; }
    string UserIpAdress { get; set; }
}

var command = request as ICommand;

if (command != null)
{
  command.AddUserid("id");
}