Is there a way to include custom fields when submitting a new sales order via the Zoho Books API?

We use something like this in the json:

{

  "customer_id":"nnnnnnnnnnnnnnnnnnn",

  "date":"2020-12-16",

  "reference_number":"OC 992",

  ........

  "custom_fields":[{"custom_field_id":"1984178000001725178","value":"OC 992"}]

}

but the Zoho Books API seems to be ignoring the custom fields.

I am posting it here, because there seems to be no available customer support at the moment from Zoho, maybe due to Covid19 lockdowns?

Juergen