To select a specific constructor, you can use this syntax:
//Create container.
var injector = new MugenInjector();
//Take empty constructor
injector.Bind<ConstructorTest>().ToSelf().TakeConstructor();
//Take constructor with string parameter
injector.Bind<ConstructorTest>().ToSelf().TakeConstructor<string>();
//Take empty constructor
injector.Bind<ConstructorTest>().To(context => new ConstructorTest());
//Take constructor with string parameter
injector.Bind<ConstructorTest>().To(context => new ConstructorTest(context.Resolve<string>()));