Add custom option Programmatically in cart its not increasing quantity in cart

Question

I am adding custom option programatically in below observer.

<event name="checkout_cart_product_add_after">
 <observer name="mymodule_checkout_cart_product_add_after" instance="ExtendStripe\Subscription\Observer\CheckoutCartProductAddAfterObserver" />
</event>

and observer code in below file its not increasing quantity in same cart item , its showing separate products in cart.

<?php
namespace ExtendStripe\Subscription\Observer;

use Magento\Framework\Event\Observer as EventObserver;
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Serialize\SerializerInterface;

class CheckoutCartProductAddAfterObserver implements ObserverInterface
{
    /**
     * @var \Magento\Framework\View\LayoutInterface
     */
    protected $_layout;
    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $_storeManager;
    protected $_request;
    
    /**
     * @var SerializerInterface
     */
    protected $serializer;
    
    /**
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\View\LayoutInterface $layout
     */
    public function __construct(
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\View\LayoutInterface $layout,
        \Magento\Framework\App\RequestInterface $request,
        SerializerInterface $serializer
    )
    {
        $this->_layout = $layout;
        $this->_storeManager = $storeManager;
        $this->_request = $request;
        $this->serializer = $serializer;
    }
    /**
     * Add order information into GA block to render on checkout success pages
     *
     * @param EventObserver $observer
     * @return void
     */
    public function execute(EventObserver $observer)
    {
        /* @var \Magento\Quote\Model\Quote\Item $item */
        $item = $observer->getQuoteItem();
        $additionalOptions = array();
        $additionalOptions[] = [
                    'label' => 'Repeats Every',
                    'value' => '1 Month'
                ];
        if(count($additionalOptions) > 0)
        {
            $item->addOption(array(
                'code' => 'additional_options',
                'value' => $this->serializer->serialize($additionalOptions)
            ));
        }
        /* To Do */
        // Edit Cart - May need to remove option and readd them
        // Pre-fill remarks on product edit pages
        // Check for comparability with custom option
    }
}

Answer 1

/app/code/Namespace/Mymodule/etc/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="catalog_product_load_after">
        <observer name="set_additional_options" instance="Namespace\Mymodule\Observer\CheckoutCartProductAddAfterObserver"/>
    </event>
</config>

app/code/Namespace/Mymodule/Observer/CheckoutCartProductAddAfterObserver.php

<?php
namespace Namespace\Mymodule\Observer;
 
use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\RequestInterface;
use Magento\Framework\Serialize\SerializerInterface;
 
class CheckoutCartProductAddAfterObserver implements ObserverInterface
{
    /**
     * @var RequestInterface
     */
    protected $_request;
     
    /**
     * @param RequestInterface $request
     */
    public function __construct(
        RequestInterface $request,
        SerializerInterface $serializer
    ) {
        $this->_request = $request;
        $this->serializer = $serializer;
    }
 
    /**
     * @param \Magento\Framework\Event\Observer $observer
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        // Check and set information according to your need
        if ($this->_request->getFullActionName() == 'checkout_cart_add') { //checking when product is adding to cart
            $product = $observer->getProduct();
            $additionalOptions = [];
            $additionalOptions[] = array(
                'label' => 'Repeats Every',
                'value' => '1 Month',
            );
            $observer->getProduct()->addCustomOption('additional_options', $this->serializer->serialize($additionalOptions));
        }
    }
}