Show dialog for reference properties using c#

Hi,

Im trying to show the reference properties dialog after adding programmatically an attachment to the active model. 

Adding the attachment is working, but now i need to show the attachment properties dialog to the user. 

I tried the key-in "reference properties" like this:

  var moniker = DgnDocumentMoniker.CreateFromFileName("C:\\reference-test\\ref-1.dgn", null);
  var attachment = Session.Instance.GetActiveDgnModel().CreateDgnAttachment(moniker, "default");

   // open reference props dialog
  Session.Instance.EnqueueKeyin("reference properties " + attachment.AttachFileName);

But nothing happens

Is the keyin correct or who can I open the attachments prop window (without showing the open file dialog)?

Thanks