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\Bitrix\Main\EventManager::getInstance()->addEventHandler('documentgenerator', 'onBeforeProcessDocument', function($event)
{
   echo '<pre>';
   $document = $event->getParameter('document');

   $fields = $document->getTemplate()->getFields();
   print_r($fields);
   die;
});

print_r($document->getFields()['MY_COMPANY']);

Array
(
    [TITLE] => Моя компания
    [VALUE] => Array
        (
            [0] => Array
                (
                    [VALUE] => 11
                    [TITLE] => ИП Копытов
                    [SELECTED] => 1
                )

            [1] => Array
                (
                    [VALUE] => 15
                    [TITLE] => КружкаСервис
                    [SELECTED] => 
                )

        )

    [GROUP] => Моя компания
)

$document->setValues(['MY_COMPANY' => 15]);