To upload a custom list of postal codes, you just need to have a file with your desired locations at hand.
Copying from a spreadsheet works just fine, as long as your postal codes are on a single line, or comma separated.
For more info on how to format your postal codes list, see examples below:
One code per line
90210, 90211, 90212
90210; 90211; 90212
Please note that a maximum of 300 codes are permitted in a single upload, while the total number of locations cannot exceed 500.