Purchase order "NB Create consistency to make the "Reason for Ord" required

Hello experts

In our company, we use the transaction ME21N to create purchase orders type NB which is a routine procedure.

When we create the purchase order, we would like that the field "Reason for Ord." on the "Retail" was mandatory according to the "Purch. Group" informed.

This procedure is ideal for buyers do not forget to fill in this field.

Can anyone tell if there is any note or improved to enable this consistency?

Thanks a lot

Ricardo Rangel