Import contract purchase agreement:INTERFACE_HEADER_ID defining

Hi, I try to create WEB ADI integrator for import contract purchase agreement.

I create  table interface, table is po_headers_interface. I need INTERFACE_HEADER_ID attribute was filled automatically with sequence PO_HEADERS_INTERFACE_S.NEXTVAL. I define this attribute: default type - SQL Query, default value - SELECT PO_HEADERS_INTERFACE_S.NEXTVAL from dual, but in Excel file all lines have the same value INTERFACE_HEADER_ID. Reccomend me please how can I difine this attribute INTERFACE_HEADER_ID  then PO_HEADERS_INTERFACE_S.NEXTVAL value was generated for every line in Excel file.