In App.OnStartup() have something like this:
ObjectFactory.Initialize(x =>
x
.ForRequestedType<ViewModel.IViewModelManager>()
.CacheBy(InstanceScope.Singleton)
.TheDefault.Is.OfConcreteType<ViewModel.ViewModelManager>()
);
Use the requested type like this:
var manager = ObjectFactory.GetInstance<IViewModelManager>();
manager.Register(this);
This pattern is most quick and most dirty.