String as a Model

I know you've already accepted an answer here - I'm adding this because there's a general gotcha associated with using a string model.

String as a model type in MVC is a nightmare, because if you do this in a controller:

string myStringModel = "Hello world";
return View("action", myStringModel);

It ends up choosing the wrong overload, and passing the myStringModel as a master name to the view engine.

In general it is easier simply to wrap it in a proper model type, as the accepted answer describes, but you can also simply force the compiler to choose the correct overload of View() by casting the string to object:

return View("action", (object)myStringModel);

The other issue you're having here of using TextBoxFor having issues with an 'unnamed' model - well you shouldn't be surprised by that... The only reason to use TextBoxFor is to ensure the fields are named correctly for binding when the underlying value is a property on a model type. In this case there is no name, because you're passing it as a top-level model type for a view - so you it could be argued that you shouldn't be using TextBoxFor() in the first place.

There's a much cleaner way of passing a string as a model into your view. You just need to use named parameters when returning your view:

[HttpGet]
public ActionResult Index()
{
    string myStringModel = "I am passing this string as a model in the view";
    return View(model:myStringModel);
}

Either wrap the string in a view model object:

Model:

public class HomeViewModel
{
    public string CodeText { get; set; }
}

Controller:

private HomeViewModel _model;

public HomeController()
{
    _model = new HomeViewModel { CodeText = "My Text" };
}

[HttpGet]
public ActionResult Index()
{
    return View("Index", _model);
}

View:

@Html.TextBoxFor(m => m.CodeText);    

Or use EditorForModel:

@Html.EditorForModel()