post https://app.asana.com/api/1.0/custom_fields//enum_options
Creates an enum option and adds it to this custom field’s list of enum options. A custom field can have at most 500 enum options (including disabled options). By default new enum options are inserted at the end of a custom field’s list.
Locked custom fields can only have enum options added by the user who locked the field.
Returns the full record of the newly created enum option.