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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))
    Dim watermark As New TextWatermark("Test watermark", New Font("Arial", 19))

    ' Editing of the shape in Visio is forbidden
    doc.AddWatermark(watermark, New DiagramShapeSettings() With { _
         .IsLocked = True _
    })

    doc.Save()
End Using