Please help me if I can set the image/shape transparency or opacity?

[ZhouYixxx]

The problem may seem naive, I'm inserting a piece of text into an Excel sheet as a watermark by drawing it as an image, but I'd like to be able to set the image to semi-transparent. I googled it for a long time. But there doesn't seem to be a good solution.
And I've tried setting the transparency of the text, which doesn't work well either.

int opacity = 128; // 50% opaque (0 = invisible, 255 = fully opaque)

using (var graphics = Graphics.FromImage(image))
using (var font = new Font("Arial", 40))
using (var brush = new SolidBrush(Color.FromArgb(opacity, Color.Red)))
    graphics.DrawString("This is a watermark", font, brush, 0, 0);

if anyone can kindly help me? Even if it makes it clear that there is no good solution it would be nice.