The custom fields are set up under the Car Park Entity and if a custom field is uses a keyword set then this must exist before the custom field can be created.
Navigate to Admin Settings > System Config > Custom Fields.
From the Entities list, select either Assets or Car Park.
When you set the custom fields under the Car Park Entity, you can select the Visibility to be Visible on Carpark form and then the custom field will display in the Edit Car Park Form's Custom Fields tab.