The PO has been re organized over the past couple of years. I think all POs can hold packages now. Here in Davis, They will hold them at the main PO. If it is being shipped to a store front, then the main one will call.
They told me that writing on the box will not hold it since it is not official. You can of course write it, but the sender needs to put the hold on the box. The person being shipped to cannot call up the PO and have them hold it for them either.
It has to be the shipper that request it. The shipper has usually shipped incorrectly if the box is not held so once again, the shipper messes up and blames the PO for their error. Not saying that the PO does not do things wrong but they are an easy target for covering mistakes by the shipper.