Hello,
The code below fixes the issue if you are using chrome, when using Firefox it opens a complete extra tab that is not needed.
Code:
<a target="_blank" class='cbMenuItem' href='#' onclick='$("#right_frame").contents().find("#caspioform").submit();
window.open("[@app:URL_F]?Onderdeel_Nummer=[@field:Nummer]&ID=[@ID]&Type=[@field:Type]&Onderdeel=[@field:Onderdeel]&Aantal=[@field:Aantal]&Prijs_Onderdeel=[@field:Prijs_Onderdeel]&Korting_Percent=0&Datum=[@field:Datum_prijslijst]","mywindow", "menubar=1,resizable=1,width=2,height=2");
'
>ADD</a>