How do I add an ISRC code onto an audio file?

[Florian]

You can use the ISRC field name at Extended Tags. If you want to add the ISRC field to the Tag Panel or the File List, you can check

Customization of Tag Panel and File List

Mp3tag supports flexible tagging, which means that the tag fields available on the Tag Panel on the left and as columns in the File List are not fixed. While you can reference any field which is, e.g., listed in the extended tag dialog as name via a placeholder %name% in Mp3tag, it's also possible to customize the UI using these placeholders. Customizing the Tag Panel The Tag Panel is the sidebar on the left of the File List that shows the fields to enter metadata. Besides the standard fields…