code: def RegisterFreeformKey(cls, key, name, mean="com.apple.iTunes"):
          
          atomid = "----:" + mean + ":" + name
  
          def getter(tags, key):
              return [s.decode("utf-8", "replace") for s in tags[atomid]]
  
          def setter(tags, key, value):
              encoded = []
              for v in value:
                  if not isinstance(v, text_type):
                      if PY3:
                          raise TypeError("%r not str" % v)
                      v = v.decode("utf-8")
                  encoded.append(v.encode("utf-8"))
              tags[atomid] = encoded
  
          def deleter(tags, key):
              del(tags[atomid])
  
          cls.RegisterKey(key, getter, setter, deleter)
docs: Register a text key.
  
          If the key you need to register is a simple one-to-one mapping
          of MP4 freeform atom (----) and name to EasyMP4Tags key, then
          you can use this function::
  
              EasyMP4Tags.RegisterFreeformKey(
                  "musicbrainz_artistid", "MusicBrainz Artist Id")