You could see these fields in the help:
and then you can add them with an action (see the FAQs: Actions and batch-operations - #15 by Florian)
or enter them manually in ...
... the extended tags dialogue (Alt-T)
... newly created columns (see the help: https://docs.mp3tag.de/customization/file-list)
... added fields in the tag panel (see the help: https://docs.mp3tag.de/customization/tag-panel)