Solve Transparent/Black background BMP image file generate via C# code

I create to design membership card with data(memebership ID, member photo, Firstname,lastnema, expire dateand level) via using CanvasDevice to draw all text with position declaration. after i generate to save (BMP) image to local storage, the stored image will like fully black background. I am not set any background and set transparent color but image shows like that.. i need a BMP image, sample code is below

int dpi = 96; // DPI (dots per inch)

        int cardWidth = 648;

        int cardHeight = 1016;

        CanvasDevice device = CanvasDevice.GetSharedDevice();

        CanvasRenderTarget renderTarget = new CanvasRenderTarget(device, cardWidth, cardHeight, dpi);

        // Clear the canvas with transparent color

        using (CanvasDrawingSession ds = renderTarget.CreateDrawingSession())

        {

            // Draw text

            ds.DrawText($"Test Data", 233, 200, Windows.UI.Colors.Black, new CanvasTextFormat { FontSize = 40, FontFamily = "Helvetica Neue" });

        }

        // Save the image to local storage as a BMP file

        StorageFolder printingCardsFolder = await storageFolder.CreateFolderAsync("resources", CreationCollisionOption.OpenIfExists);

        StorageFile file = await printingCardsFolder.CreateFileAsync($"Test_Card.bmp", CreationCollisionOption.ReplaceExisting);

        using (IRandomAccessStream fileStream = await file.OpenAsync(FileAccessMode.ReadWrite))

        {

            //BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.BmpEncoderId, fileStream);

            //encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore,

            //    (uint)cardWidth, (uint)cardHeight,

            //    dpi, dpi,

            //    canvasBitmap.GetPixelBytes());

            //await encoder.FlushAsync();

            await renderTarget.SaveAsync(fileStream, CanvasBitmapFileFormat.Bmp);

        }

        // Set the saved image as the source of MembershipCardImage

        BitmapImage bitmapImage = new BitmapImage(new Uri(file.Path));

        MembershipCardImage.Source = bitmapImage;