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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 DiagramDocument = Document.Load(Of DiagramDocument)(Utilities.MapSourceFilePath(FilePath))
    ' Initialize watermark of any supported type
    Dim textWatermark As New TextWatermark("Test watermark 1", New Font("Calibri", 19))

    ' Create separate background for each page where it is not set. Add watermark to it.
    doc.AddWatermark(textWatermark, DiagramWatermarkPlacementType.SeparateBackgrounds)

    doc.Save()
End Using