Extend /sitecore/api/ssc/EXM/Message with additional data

I want to extend /sitecore/api/ssc/EXM/Message endpoint with additional data.

I added a new class

namespace Exm.Extended
{
    [ServicesController("EXM.Message")]
    public class MessageController : Sitecore.EmailCampaign.Server.Controllers.Message.MessageController
    {
        //

        //private ParentController _parent;
        public MessageController(
            ItemUtilExt itemUtil,
            ILanguageRepository languageRepository,
            IMessageVariantsService messageVariantsService,
            ILogger logger,
            IExmCampaignService exmCampaignService)

            : base(itemUtil, languageRepository, messageVariantsService, logger, exmCampaignService)
        {
           
        }

       
        [ActionName("DefaultAction")]
        public new Response Message( MessageContext data )
        {
            MessageResponse response = (MessageResponse) base.Message(data);
           //todo change/extend response
            return response;
        }
    }
}

I added a custom DI configurator

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <services>
      <configurator type="EXM.Custom.Configurator, My.Website"/>
...

and Replaced the default EXM MessageController with my custom one

 public void Configure(IServiceCollection serviceCollection)
 {
container.Replace(ServiceDescriptor.Transient(typeof(Sitecore.EmailCampaign.Server.Controllers.Message.MessageController), typeof(Exm.Extended.MessageController)));
..

if I debug now, the custom.messagehandler constructor is hit, but the new default action does never run => still always the Sitecore.EmailCampaign.Server.Controllers.Message.MessageController.Message action.