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$couponResult = null;

$saleDiscountDb = \CSaleDiscount::GetList(
   array(
      'DATE_CREATE' => 'DESC'
   ),
   array(
      'XML_ID' => 'MY_XML',
      'ACTIVE' => 'Y'
   ),
   false,
   false,
   array(
      'ID',
      'ACTIONS',
      'CONDITIONS'
   )
);

$saleDiscountId = null;
if ($saleDiscount = $saleDiscountDb->Fetch()) {
   $saleDiscountId = $saleDiscount['ID'];
}

if ($saleDiscountId) {
   $coupon = \Bitrix\Sale\Internals\DiscountCouponTable::generateCoupon(true);

   $addDb = \Bitrix\Sale\Internals\DiscountCouponTable::add(array(
      'DISCOUNT_ID' => $saleDiscountId,
      'COUPON'      => $coupon,
      'TYPE'        => \Bitrix\Sale\Internals\DiscountCouponTable::TYPE_ONE_ORDER,
      'MAX_USE'     => 1,
      'USER_ID'     => 0,
      'DESCRIPTION' => ''
   ));

   if ($addDb->isSuccess()) {
      $couponResult = $coupon;
   }
}

return $couponResult;