Program.cs

for (var i = 0; i <= 100; i++)
{
    var putItemRequest = new PutItemRequest
    {
        TableName = sourceTableName,
        Item = new Dictionary<string, AttributeValue>()
        {
            {"City", new AttributeValue{S=cities[(new Random()).Next(cities.Length)]  } },
            {"Date", new AttributeValue{S=GetRandom2020Date()  } },
            {"Highest", new AttributeValue{N=(new Random()).Next(20,30).ToString()  } },
            {"Lowest", new AttributeValue{N=(new Random()).Next(1,10).ToString()  } }
        }
    };
    await client.PutItemAsync(putItemRequest);
}