Draw Text on Transparent Image - How to draw text on transparent image?

using (var bitmap = new Bitmap(Convert.ToInt32(Width.Value), Convert.ToInt32(Height.Value), PixelFormat.Format32bppArgb, new RgbColor(255, 255, 255, 255)))
{
    using (var graphics = bitmap.GetAdvancedGraphics())
    {
        using (var fontRegistry = new CustomFontRegistry())
        {
            var brush = new SolidBrush(color);
            var customFont = CustomFontCollection.TryGetAurigmaFont(Font);
            float dpi = 300;

            BoundedText text = null;

            Aurigma.GraphicsMill.AdvancedDrawing.Font font = null;

            if (!string.IsNullOrEmpty(customFont))
            {
                var psName = fontRegistry.Add(customFont);
                font = fontRegistry.CreateFont(psName, (float)FontSize, 72, 72);
                dpi = font.DpiX;
            }
            else
            {
                font = graphics.CreateFont(Font, (float)FontSize);
            }

            
            text = new BoundedText(Text, font, new System.Drawing.RectangleF(0, 0, (float)Width.Value, (float)Height.Value));
            text.Pen = new Pen(color);

            graphics.DrawText(text);
            _RenderedFontGraphicsFile = ctx.GetPathToNewTempFile("hi.png");
            bitmap.Save(_RenderedFontGraphicsFile);
        }
    }
}