add a bookmark in a word document - WordprocessingML - Formats

Hi, I tried to add a bookMarkStart and a bookMarkEnd in a document, removing hidden text.

My code:

Public Class ManipulateWordProcessingDocument

    Private _wdDoc As WordprocessingDocument

    Public Sub New(ByVal wDoc As WordprocessingDocument)
        Me._wdDoc = wDoc
    End Sub

    Public Function FindHiddenText(ByVal textoOculto As String) As Run
        Dim runResult As Run = Nothing
        Try
            runResult = Me._wdDoc.MainDocumentPart.RootElement.Descendants(Of Run).Where(Function(r) r.RunProperties IsNot Nothing AndAlso
                            r.RunProperties.Vanish IsNot Nothing AndAlso r.InnerText.Equals(textoOculto)).FirstOrDefault
        Catch ex As Exception
            If True Then

            End If
        End Try

        Return runResult
    End Function

    ''' <summary>
    ''' Devuelvo la marca final porque entre el marcador BookMarkStart y el BookMarkEnd, nosotros no tenemos nada, y siempre
    ''' nos va a interesar hacer cualquier cosa despues del BookMarkEnd, y siempre comparten padre.
    ''' </summary>
    ''' <param name="nombreMarca"></param>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Public Function FindBookMark(ByVal nombreMarca As String) As BookmarkEnd
        Dim bookmark As BookmarkEnd = Nothing
        Dim marcaStart As BookmarkStart
        Try
            marcaStart = Me._wdDoc.MainDocumentPart.RootElement.Descendants(Of BookmarkStart).Where(Function(b) b.Name.Value.Equals(nombreMarca)).FirstOrDefault
            If marcaStart IsNot Nothing Then
                bookmark = bookmark.Parent.Descendants(Of BookmarkEnd).Where(Function(b As BookmarkEnd) b.Id.Value.Equals(marcaStart.Id.Value)).FirstOrDefault
            End If
        Catch ex As Exception
            If True Then

            End If
        End Try

        Return bookmark
    End Function

Public Function ReplaceHiddenTextByBookMark(ByVal nombreOculto As String) As Boolean
        Dim oculto As Run = Nothing
        Dim elementoPadre As OpenXmlElement = Nothing
        Dim marcadorStart As BookmarkStart = Nothing
        Dim marcadorEnd As BookmarkEnd = Nothing

        oculto = Me.FindHiddenText(nombreOculto)

        If oculto IsNot Nothing Then
            Try
                elementoPadre = oculto.Parent

                marcadorStart = New BookmarkStart()
                marcadorStart.Name = "Principio" & nombreOculto
                elementoPadre.InsertAfter(marcadorStart, oculto)
                marcadorEnd = New BookmarkEnd()
                marcadorEnd.Id = marcadorStart.Id
                elementoPadre.InsertAfter(marcadorEnd, marcadorStart)

                marcadorStart = New BookmarkStart()
                marcadorStart.Name = "Fin" & nombreOculto
                elementoPadre.InsertAfter(marcadorStart, marcadorEnd)
                marcadorEnd = New BookmarkEnd()
                marcadorEnd.Id = marcadorStart.Id
                elementoPadre.InsertAfter(marcadorEnd, marcadorStart)

                oculto.Parent.RemoveChild(oculto)
                Return True
            Catch ex As Exception
                If True Then

                End If
            End Try
        End If
        Return False
    End Function

....

End Class

But the property id of instance "marcadorStart" is nothing, how I do to create a instance of BookMarkStart with id?