'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")'

@classmethod
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)
