Welcome to OpenStreetMap.

If you want to add a postcode to a street, or another object which isn't directly addressable, you can add a postal_code=* tag. For streets, it's more common to just add the postal district as addresses may have different postcodes, but with a short street where all the addresses have the same postcode, the full postcode is fine.

Addressable objects like houses and businesses use addr:postcode=* instead.

https://wiki.openstreetmap.org/wiki/Key:postal_code