use item name by an item identifier (item with the id itemIdentifier should exist).
{ "id":"_ID_", "method":"hub.scenes.expressions.set", "params":{ "name": "HalfOfSoundLevel", "code": "return params.sound_level / 2", "params": { "items": [ { "name": "sound_level", "_id": "sirenSoundLevelItemIdentifier"} ] } } }