This is my old implementation to get a Unique DeviceID for Windows Universal 8.1 but the type HardwareIdentification does not exist anymore.
    private static string GetId()
    {
        var token = HardwareIdentification.GetPackageSpecificToken(null);
        var hardwareId = token.Id;
        var dataReader = Windows.Storage.Streams.DataReader.FromBuffer(hardwareId);
        byte[] bytes = new byte[hardwareId.Length];
        dataReader.ReadBytes(bytes);
        return BitConverter.ToString(bytes).Replace("-", "");
    }