Hi @willAM42F. There are two main ways to 'attach' a dimension to an Inventor.TextBox. The first, and simplest is by using its TextBox.OriginSketchPoint. But as it says in its documentation, it will only exist if you have not displayed the boundaries of the TextBox. The other way is by showing the boundary, then getting the geometry of that boundary, and using those as what to attach your dimensions to.
TextBox.ShowBoundaries
TextBox.BoundaryGeometry
The documentation for that last one does not mention it, but I would assume that you would not get anything from the TextBox.BoundaryGeometry if you have not yet set the TextBox.ShowBoundaries property to True yet. Beyond that, I assume you already know how to get/create GeometryIntent objects from those geometry objects for use with your dimension creation codes.
Wesley Crihfield

(Not an Autodesk Employee)