I set the target property of the animation using the fully-qualified dependency property name:
Storyboard.SetTargetProperty(anima, new PropertyPath(RotateTransform.AngleProperty));
The alternative is using a string:
Storyboard.SetTargetProperty(anima, new PropertyPath("Angle"));
You might prefer that syntax because it’s shorter, but it doesn’t guarantee that you haven’t misspelled the property name.
The advantage of using strings for property names is that you can stack property names in a pile. This allows you to animate a property of an object without referencing that object itself. For example, in the Click event handler above you can set the target of the animation to be the Button instead of the RotateTransform: