let responseJson = await result.json();
　↓
}
Object {
  "replies": Array [
    Object {
      "addSheet": Object {
        "properties": Object {
          "gridProperties": Object {
            "columnCount": 26,
            "rowCount": 1000,
          },
          "index": 8,
          "sheetId": 162165881,
          "sheetType": "GRID",
          "title": "bk8",
        },
      },
    },
  ],
  "spreadsheetId": "1xOspY16-yGAa3WNWovLMhFexV4MsFT1RCGOVr_B-sbg",
}


// sheetId
ret = responseJson.replies[0].addSheet.properties.sheetId