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' For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
Using doc As Document = Document.Load(Utilities.MapSourceFilePath(PngFilePath))
    Dim font As New Font("Calibri", 12)
    Dim watermark As New TextWatermark("This is a test watermark", font)

    ' Set sizing type
    watermark.SizingType = SizingType.ScaleToParentDimensions

    ' Set watermark scale
    watermark.ScaleFactor = 0.5

    doc.AddWatermark(watermark)
    doc.Save()
End Using