<?xml version="1.0" encoding="utf-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="email_order_set_template_vars_before">
        <observer name="add_Custom_variable_to_Order"
            instance="[Vendor][ModuleName]ObserverObserverforAddCustomVariable" />
    </event>
</config>

<?php
namespace [Vendor][ModuleName]Observer;
use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkAppRequestDataPersistorInterface;
use MagentoFrameworkAppObjectManager;

class ObserverforAddCustomVariable implements ObserverInterface
{

    protected $ruleFactory

    public function __construct(MagentoSalesRuleModelRuleFactory $ruleFactory) {
        $this->rulesFactory = $ruleFactory;
    }

    /**
     *
     * @param MagentoFrameworkEventObserver $observer
     * @return void
     */
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        /** @var MagentoFrameworkAppActionAction $controller */
        $transport = $observer->getTransport();
        $couponCode = $this->createOneCoupon();
        if($couponCode){
            $transport['free-coupon'] = $couponCode;
        }
    }
        protected function createOneCoupon()
    {
        $ruleModel = $this->ruleFactory->create();
        $ruleModel->load({RulesID});
         try {
                $data = array(
                        'rule_id' => 1,
                        'qty' => 1,
                        'length' => '12',
                        'format' => 'alphanum',
                        'prefix' => 'free-shipping',
                        'suffix' => '',
                        'dash'=>0
                        );


                /** @var $generator MagentoSalesRuleModelCouponMassgenerator */
                $generator = $this->_objectManager->get('MagentoSalesRuleModelCouponMassgenerator');
                if (!$generator->validateData($data)) {
                   return false;
                } else {
                    $generator->setData($data);
                    $generator->generatePool();
                    $generated = $generator->getGeneratedCount();
                    $codes = $generator->getGeneratedCodes();
                    return $codes[0];

                }
            } catch (MagentoFrameworkExceptionLocalizedException $e) {
                $this->_objectManager->get('PsrLogLoggerInterface')->critical($e);
                return false;
            } catch (Exception $e) {

                $this->_objectManager->get('PsrLogLoggerInterface')->critical($e);
                return false;
            }
    }

}