Your account may create apps, which contain custom fields, through the Update orderForm configuration request. The value of a specific custom field can then be updated by this request.
To do that, you need to inform in the URL the ID of the app you created with the configuration API (appId).
In the body of the request, you will inform the new value (appFieldValue, passed through the body) of the specific field created in this app (identified by the appFieldName parameter, passed through the URL).
The orderForm is the data structure which represents a shopping cart and contains all information pertaining to it. Hence, the orderFormId is the identification code of a given cart.
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