matrix-spec/data/event-schemas/examples/m.reaction.yaml

{
    "$ref": "core/room_event.json",
    "type": "m.reaction",
    "content": {
        "m.relates_to": {
            "rel_type": "m.annotation",
            "event_id": "$some_event_id",
            "key": "👍"
        }
    }
}