How to get StructureMap to return a specific instance for a requested type

var myObj = ObjectFactory.GetInstance(typeof(MyAbstractClass));

var myClass = new MyConcreteClass("bla"); // MyConcreteClass : MyAbstractClass

myObj == myClass

ObjectFactory.Initialize(x =>
        {
            x.ForRequestedType<MyAbstractClass>().TheDefault.IsThis(myClass);
        });

ObjectFactory.Inject(typeof(MyAbstractClass), myClass);