Creates a custom field for your organization. The field is scoped to the organization resolved from your API key; any organizationId in the body is ignored. fieldKey must be unique among your active fields, and type is fixed once created. When hasFixedOptions is true, allowedValues is required and non-empty; when false, allowedValues must be empty or omitted. displayAs: AGE shows a DATE field as an age; it is rejected on any other type, and omitting it defaults to DATE.

Capacity: there is a maximum number of fields per type, and deleting a field does not free that capacity (deletes are permanent). Provision fields deliberately. A create that exceeds the limit for a type returns 400.