There are a few ways to extend when you originally scanned a QR code.
The first is to open back up your browser on your phone that was opened to pay through HONK. You should see a pink 'Extend Session' link that you can tap to initiate the extension.
Another way to extend is to wait for the expiry notification and use the extension link.
You would need to enter your phone number after completing your purchase for a notification, which is an extra step after entering your email.
Otherwise you can download the app, create an account and make a purchase at the zone in question when the original purchase ends. The zone will be listed on your receipt and on the success screen once you have completed your purchase.
Or you can let us know (by emailing support) the vehicle you paid for or even the initial zone and we can send the same express checkout link you purchased through originally.