Add Text Watermark to RTF Using C#. For more information, please follow link: https://kb.groupdocs.com/watermark/net/add-text-watermark-to-rtf-using-csharp/

Add Text Watermark to RTF Using C#.cs

using GroupDocs.Watermark.Common;
using GroupDocs.Watermark.Watermarks;
using GroupDocs.Watermark;

namespace AddTextWatermarktoRTFUsingCSharp
{
    internal class Program
    {
        static void Main(string[] args)
        {
            // Set License to avoid the limitations of Watermark library
            License lic = new License();
            lic.SetLicense(@"GroupDocs.Watermark.lic");

            // Specify an absolute or relative path to your RTF
            using (Watermarker watermarker = new Watermarker("input.rtf"))
            {
                // Specify the desired text and font for the watermark
                TextWatermark watermark = new TextWatermark("Watermark Text", 
                                              new Font("Arial", 60, FontStyle.Bold));
                // Specify font color and text opacity, rotation and alignments
                watermark.ForegroundColor = Color.DarkGreen;
                watermark.Opacity = 0.5;
                watermark.HorizontalAlignment = HorizontalAlignment.Center;
                watermark.VerticalAlignment = VerticalAlignment.Center;
                watermark.RotateAngle = -45;
                // Apply the watermark
                watermarker.Add(watermark);
                // Save the resulting RTF 
                watermarker.Save("output.rtf");
            }
        }
    }
}