Open form to add record with fielddata from an open form

How do I open a form to add a record and get the data for 1 field from a
specific field in a open form.

I have used the following code ;

Private Sub Form_Open(Cancel As Integer)

If IsOpen("FrmStam") Then
DebNr.DefaultValue = _
Forms!FrmStam!DebNr
End If

End Sub

When I open the add record form the field DebNr is filled with [#Name?#]
Where do I go wrong ?