Look at the batch number group and then assign it to the item. Seems possible from what you have said, reference number and number sequence on inventory transaction. I see you have the date in there and you would need to verify if what it picks meets your needs, but arguably having the date is redundant depending upon what you are using it for.
Look at the setup of the batch number group itself - on the general tab there is a structure section, in there is a reference to the number sequence in teh format. Be aware there is a limit to this field size, so the combonaiton of PO, Date and Number Sequence must be below it.
As stated the batch number group has a structure that you define on the general tab. You need to go to the actual batch number group record and look at its setup.