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\Bitrix\Main\Loader::includeModule('iblock');

$property = \CIBlockProperty::GetList(
    [],
    [
        'IBLOCK_ID' => $iblockId,  
        'CODE' => $code'
    ]
)->Fetch();

$ibpenum = new \CIBlockPropertyEnum();
$valueId = $ibpenum->Add([
    'PROPERTY_ID' => $property['ID'],
    'VALUE' => $newValueText,
    'XML_ID' => $newValueXmlId,
]);
if ((int) $valueId < 0) {
    throw new \Exception('Unable to add a value');
}