How can I print to a specific paper tray ?

procedure PrintPDF(APrinterIndex: Integer; AFileName: String; APrintTitle: String);

var

  vForm: TForm; 

  vWpViewPDF: TWPViewPDF;

begin

  vForm := TForm.Create(nil);

  vForm.Visible := False;

  vWpViewPDF := TWPViewPDF.Create(vForm);

  vWpViewPDF.Parent := vForm;

  try

    vWpViewPDF.filename := AFileName;

    vWpViewPDF.CommandStr(COMPDF_SetPrintTitle, APrintTitle);

    vWpViewPDF.command(COMPDF_SetPagePaperBin, 39);

    vWPViewPDF.BeginPrint(Printer.Printers[APrinterIndex]);

    vWPViewPDF.PrintPages(1, vWpViewPDF.PageCount);

    vWPViewPDF.EndPrint;

  finally

    FreeAndNil(vWpViewPDF);

    FreeAndNil(vForm);

  end;

end;