Beta. You can define and manage custom fields now. Populating candidate values and filtering or searching by them arrives in a later release.

Updates a custom field's label, hasFixedOptions and/or allowedValues. At least one field must be provided. type is immutable and cannot be changed. hasFixedOptions and allowedValues must stay consistent (a fixed-options field needs non-empty allowedValues; a free-input field must have none). A field belonging to another organization, or one that has been deleted, returns 404.