Have you tried with standard.
I hope you have made these fields as displayed mode from field selection.
Make the field selection as option for PO quantity.
Then you have to do nothing. Just go to ME21N or ME22N, add a new schedule line in tab delivery schedule.
Your PO quantity will be displayed mode. Its a standard behavior.
Once you will use more than one line item in delivery tab. Then the field quantity (po line item) will come display mode.
