Make PO Box City field as required when either of other fields r non-blank

Hi,

I need your help and valuable suggestion to fix this problem.

While Creating/Changing a vendor, I would like to input some data in the PO box address area (Table LFA1). If I populate the fields PO Box & Postal code fields in this area, the PO box City field should become a required one. Currently it is an optional field. I dont want to do it at field status level.

If there are not any data in either of those fields, then PO box city field should be an optional one.

How can we do this? Please provide me a solution in detail.