catalog_product_load_after:

public function execute(MagentoFrameworkEventObserver $observer) {
        // Check and set information according to your need
        if ($this->_request->getFullActionName() == 'checkout_cart_add') { //checking when product is adding to cart
            $post = $this->_request->getParam('personalized');

            $additionalOptions = [];
            $additionalOptions[] = array('label' => "labelVal1", 'value' => 'Text1');
            $additionalOptions[] = array('label' => "labelVal2", 'value' => 'Text2');

            if (count($additionalOptions) > 0) {
                $observer->getProduct()->addCustomOption('additional_options', serialize($additionalOptions));
            }
        }
    }

$quoteObj = $this->_cart->getQuote();

                    $item_id = 201;
                    $item = $quoteObj->getItemById($item_id);


                    if (!empty($item)) {


                        $additionalOptions = [];


                        ### Setting up additional options
                        $additionalOptions[] = array('label' => "labelVal1", 'value' => 'Text3');
                        $additionalOptions[] = array('label' => "labelVal2", 'value' => 'Text4');


                        if (count($additionalOptions) > 0) {

                            $item->getProduct()->addCustomOption('additional_options', serialize($additionalOptions));
                        }

                        $item->getProduct()->setIsSuperMode(true);
                        $item->save();
                    }