A post office box address (P.O. Box) in the United States cannot be used as your "voting residence address."
When you register to vote and request an overseas absentee ballot, your local election official in the United States needs your last U.S. residence address in order to determine your precise voting district. That is where your ballot request will be processed and where your ballot will be counted. Your voting residence address must be a physical street or rural route address, or a location that you describe in precise detail.