patch https://partner-api.getpliant.com/api/cards//custom-fields
Update the default values of custom fields related to a card or add new ones by label. All custom fields need to be provided, by not providing a custom field, the default value of that custom field will be removed, i.e. set to null. Updates of existing custom fields may happen via id or label. Adding new fields is only possible by providing a currently unused label. Note: the label needs to be unique per organization.