You need a custom tag, not a virtual tag, for this task.
1. Go to Preferences> Tags (1)> Define new tags
2. Enter "Catalognumber" in identifier box and press tab to populate it in other boxes.
3. Save and close the dialog and return to Tags (1) dialogue.
4. For custom4 tag, put "Catalog" or something in "display name" and select "Catalognumber" for "Save to..".
5. Then save the dialog, now you will see the values populate in Catalog field.