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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (MultiframeImageDocument doc = Document.Load<MultiframeImageDocument>(FilePath))
{
    // Initialize text or image watermark
    TextWatermark watermark = new TextWatermark("Test watermark", new Font("Arial", 19));

    // Add watermark to the first frame
    doc.Frames[0].AddWatermark(watermark);

    doc.Save();
}