Adding text watermark | https://docs.conholdate.com/net/adding-text-watermark/

AddTextWatermark.cs

// Constants.InImagePng is an absolute or relative path to your document. Ex: @"C:\Docs\image.png"
using (Watermarker watermarker = new Watermarker(Constants.InImagePng))
{
    // Initialize the font to be used for watermark
    Font font = new Font("Arial", 19, FontStyle.Bold | FontStyle.Italic);

    // Create the watermark object
    TextWatermark watermark = new TextWatermark("Test watermark", font);

    // Set watermark properties
    watermark.ForegroundColor = Color.Red;
    watermark.BackgroundColor = Color.Blue;
    watermark.TextAlignment = TextAlignment.Right;
    watermark.Opacity = 0.5;

    // Add watermark
    watermarker.Add(watermark);
    watermarker.Save(Constants.OutImagePng);
}

Read the complete article: https://docs.conholdate.com/net/adding-text-watermark/