DataMatrix FNC1 at first byte

[NenashevArtem]

I am using the ZXing.Net package (0.16.11) in a .NET9 project. I need to generate a DataMatrix GS1 image. I need the symbol, when read, to have the string start with the FNC1 code (0xE8), but I can't get this to work. Setting GS1Format = true doesn't help. Here is my method:

public static Bitmap GenerateFromBase64(int size, string base64Data)
{
    byte[] rawBytes = Convert.FromBase64String(base64Data);

    // Add prefix 0xE8 if not exist
    byte[] finalBytes;
    if (rawBytes.Length > 0 && rawBytes[0] == 0xE8)
    {
        // Prifix exist
        finalBytes = rawBytes;
    }
    else
    {
        // Add prefix
        finalBytes = new byte[rawBytes.Length + 1];
        finalBytes[0] = 0xE8;
        Buffer.BlockCopy(rawBytes, 0, finalBytes, 1, rawBytes.Length);
    }

    string data = System.Text.Encoding.GetEncoding("ISO-8859-1").GetString(rawBytes);

    var writer = new BarcodeWriterPixelData
    {
        Format = BarcodeFormat.DATA_MATRIX,
        Options = new DatamatrixEncodingOptions
        {
            Height = size,
            Width = size,
            Margin = 0,
            SymbolShape = SymbolShapeHint.FORCE_SQUARE,
            GS1Format = true,
            PureBarcode = false
        }
    };

    var pixelData = writer.Write(data);

    var bitmap = new Bitmap(pixelData.Width, pixelData.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    var bitmapData = bitmap.LockBits(new Rectangle(0, 0, pixelData.Width, pixelData.Height),
                                     System.Drawing.Imaging.ImageLockMode.WriteOnly,
                                     System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    try
    {
        System.Runtime.InteropServices.Marshal.Copy(pixelData.Pixels, 0, bitmapData.Scan0, pixelData.Pixels.Length);
    }
    finally
    {
        bitmap.UnlockBits(bitmapData);
    }
    return bitmap;
}

Testing with value

string base64Input = "MDEwNDYwMTAzMjAxMDkwNDIxNVpTRmZHUUxRckhoNkRjKkdZeU0dOTFFRTEyHTkyZmw1akJyTGRoeUNLUklmMHdwWEZ5eExUZ3BqWEtBQzJWQzZKM01jY1N6Yz0=";
Bitmap dmCode = DataMatrixTest.DataMatrixGenerator.GenerateFromBase64(300, base64Input);

It mast by like this, but no e8 at begining

[NenashevArtem]

It seems that solving problem here https://github.com/micjahn/ZXing.Net/wiki/Generating-GS1-DataMatrix-and-GS1-Code128-barcodes