At the USPS website, at this link.....Click the "General Delivery" tab in the grey horizontal menu after the first paragraphs about forwarding:
General Delivery
Get your mail even if you’re on the road, new to town, or between permanent addresses. Mail addressed to you at General Delivery will be held at the area’s main Post Office™ for up to 30 days. All you have to do is pick it up.
General Delivery is a great choice if you don’t have a permanent address. People can send you mail by using the town name and ZIP Code™, like this...JOHN DOE
GENERAL DELIVERY
ANYTOWN NY 12345-9999
In medium to large cities with multiple ZIP Codes, you’ll want to make sure senders use the ZIP Code for the area’s main Post Office. The ZIP+4® extension 9999 indicates general delivery.
To find the main Post Office in an area, speak to any Post Office associate or call 1-800-ASK-USPS® (1-800-275-8777).
https://www.usps.com/manage/forward-mail.htm