{
        "createdBy": "student",
        "createdOn": "2019-06-18",
        "Id1": "0e8b9445-4bd9-4d31",
        "Tl": [
            {
                "createdBy": "student",
                "createdOn": "2019-06-18",
                "Id2": "d46eeb88-f876-4468"
            }
        ]
    }

var jsonData = JSON.parse(responseBody);
postman.setEnvironmentVariable("Id1", jsonData.Id1);
postman.setEnvironmentVariable("Id2", jsonData.Tl[2].Id2);