There are a couple of ways to do it:
- Enhance the CR genil model. Go to transaction GENIL_MODEL_BROWSER. Then, create an enhancement for genil model CR. In the enhancement, use a new custom handler class. This custom handler class must be a child of the original handler class. In there, redefine the GET_ATTRIBUTES method to default the values as you like. Make sure you call the super method always before adding your own logic in the redefined method.
- Enhance the CR Header feeder class GET_DATA method. In there, you can always check if the description is blank and if so set it to the change number field.