Sub addBM_Header()
Dim oRng As Range
Dim oBM As Bookmark
    'Check if the bookmark exists
    For Each oBM In ActiveDocument.Bookmarks
        If oBM.Name = "NombreEncab" Then
            GoTo FillBM
            Exit For
        End If
    Next oBM
    'Bookmark not present so add-it
    Set oRng = ActiveDocument.Sections(1).Headers(wdHeaderFooterPrimary).Range
    If Len(oRng) > 3 Then
        oRng.Start = oRng.Start + 3
        oRng.Collapse 1
    End If
    ActiveDocument.Bookmarks.Add Range:=oRng, Name:="NombreEncab"

FillBM:
    'Write to the bookmark
    Set oRng = ActiveDocument.Bookmarks("NombreEncab").Range
    oRng.Text = "bookmark text"
    'Reset the bookmark
    ActiveDocument.Bookmarks.Add Range:=oRng, Name:="NombreEncab"
End Sub