Add a Created By Field on Cases

To create the custom field, perform the following steps:

1. Navigate to Customization > Lists, Records, & Fields > CRM Fields > New.
2. Provide a Label (e.g. Created By:). 
3. Set Type to Free-Form Text.
4.  Uncheck Store Value.
5. In the Applies To tab, check Case.
6. In the Display tab, set Subtab to Main.
7. In the Display tab, set Display Type to Inline Text.
8. Click the Validation and Defaulting tab.
9. Check Formula checkbox > in the Default Value box, enter {systemnotes.name}
10. Click Save.