open purchase order overview form from custom form

How can I open the po-overview form with a po_header_id from a custom form?

This works pretty fine for the standart po form but not for the overview:

fnd_function.execute (function_name => 'PO_POXPOEPO', 
                                            open_flag     => 'Y', 
                                            session_flag  => 'N', 
                                            other_params  => ' PO_HEADER_ID="'||PO_HEADER_ID||'"  ACCESS_LEVEL_CODE="VIEW_ONLY" POXPOEPO_CALLING_FORM="POXSTNOT"' );   

Any ideas?