Are you expecting to be able to set/edit PreviousText from within the Inspector when an instance of TextAreaAdv is added to a window? If so, that is not yet implemented. In Xojo, they term that "Inspector Behaviour" and is accessible by right clicking a custom class and indicating which of the properties are visible in the inspector.
This is a good feature and I've added a feature request for it: https://feedback.objo.dev/feature/138