Evert,

You will need a plain bookmark stdadv where you want the field to
appear.  As you can never tell what your users will do, it is a bit
more complicated than just having the result of the first dropdown add
a new field.  What if the user changes their mind and then changes it
again ;-)

Try this on exit from the dropdown named "tel04":

Sub optipointselekt()
Dim oFfld As FormFields
Dim oRng As Word.Range
Dim oDoc As Word.Document
Set oDoc = ActiveDocument
Set oFfld = oDoc.FormFields
Set oRng = oDoc.Bookmarks("stdadv").Range
oDoc.Unprotect
If oFfld("tel04").Result = "Tel04B" Then
 If Not ActiveDocument.Bookmarks.Exists("soortoptipoint") = True Then
   oFfld.Add Range:=oRng, Type:=wdFieldFormDropDown
   With oRng.FormFields(1)
     .Name = "soortoptipoint"
     .Enabled = True
     .OwnHelp = False
     .HelpText = ""
     .OwnStatus = False
     .StatusText = ""
   End With
   With oFfld("soortoptipoint").DropDown.ListEntries
     .Clear
     .Add "Standard"
     .Add "Advanced"
   End With
   oFfld("soortoptipoint").DropDown.Value = 1
 End If
Else
 If ActiveDocument.Bookmarks.Exists("soortoptipoint") = True Then
   oFfld("soortoptipoint").Delete
 End If
End If
ActiveDocument.Protect wdAllowOnlyFormFields, True
End Sub

I didn't extensively test, but if the second formfield already exists
then you either want to ignore adding it again or delete it if
depending on subsequent user actions.