You can mark your custom annotation with the following Scout annotation: "org.eclipse.scout.rt.platform.annotations.DtoRelevant". If a form field is then annotation with your custom annotation it will be copied to the generated DTO. Please note that your custom annotation must be on the classpath of the shared project where the DTOs are stored.
Do you add the annotation to the form fields manually? Or do you want to include your annotation automatically if the code completion (ctrl+space) is used?