// Load the Excel file
Workbook workbook = new Workbook("Excel.xlsx");

// Get the first default sheet
Worksheet sheet = workbook.getWorksheets().get(0);

// Add watermark
Shape wordart = sheet.getShapes().addTextEffect(MsoPresetTextEffect.TEXT_EFFECT_1, "CONFIDENTIAL",
        "Arial Black", 50, false, true, 18, 8, 1, 1, 130, 800);

// Get the fill format of the word art
FillFormat wordArtFormat = wordart.getFill();

// Set the color
wordArtFormat.setOneColorGradient(Color.getRed(), 0.2, GradientStyleType.HORIZONTAL, 2);

// Set the transparency
wordArtFormat.setTransparency(0.9);

// Make the line invisible
wordart.setHasLine(false);

// Lock shape aspects
wordart.setLocked(true);
wordart.setLockedProperty(ShapeLockType.SELECTION, true);
wordart.setLockedProperty(ShapeLockType.SHAPE_TYPE, true);
wordart.setLockedProperty(ShapeLockType.MOVE, true);
wordart.setLockedProperty(ShapeLockType.RESIZE, true);
wordart.setLockedProperty(ShapeLockType.TEXT, true);

// Save the watermarked Excel file
workbook.save("output.xlsx");