How to catch the car registration in a Blippa parking solution

We have just delivered a payment solution for a parking. The customer pays the minimum fee (24 hours) to get in. And then, for any additional days when leaving the parking area.
The gate is opened by using a Flexykey device with a SIM card and internet access.
A Blippa action opens the gate after a successful payment.
Since no Blippa objects are created (all parking information is stored inside a Blippa data source), we needed a way to make the car registration number searchable (for refunds etc).
The solution was to add an Extra field in the Form-Payment. By adding the the car registration from a form field to the Extra field, the registration number is included in the payment information that is stored in Blippa.