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CModule::IncludeModule("sale");

$couponList = \Bitrix\Sale\Internals\OrderCouponsTable::getList(array(
    'select' => ['COUPON'],
    'filter' => ['=ORDER_ID' => 93]
));

$coupons = [];

while ($coupon = $couponList->fetch())
{
    $coupons[] = $coupon['COUPON'];
}

$couponString =  implode(', ', $coupons );