I would like to add a field for Recording Location. Recording Location is how it is described in Roon, "recorded" at in musicbrainz. How would I create this custom tag/field in mp3tag?
Either create an action of the type "Format value" and type in the field name as field and then set the value. The same can be achieved with Convert>Tag-Tag.
Or use the extended tags dialogue Alt-T and create a new field.
If you are working with FLAC files I suggest using the tag name LOCATION, as that name is recommended here: Ogg Vorbis I format specification