Serial number generation at the time of sales order entering

We have requirement for serial number to be given at the time of sales order entering,

 as our item set up iSerial Number Generation is  Dynamic entry at inventory receipt, Sales user Enters the item code & give serial number in Line DFF, in which we have made a report it searched maximum serial number

in  open order created or form MTL Serial number & sales user add +1 of this seria number in line DFF which is a manual process & error prone

can any one guide me , how the serial number is can be given at the time of  booking .