/**
 * @param RequestInterface $request
 */
public function __construct(
    RequestInterface $request
) {
    $this->_request = $request;
}

/**
 * @param MagentoFrameworkEventObserver $observer
 */
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
        $product = $observer->getProduct();
        $additionalOptions = [];
        $additionalOptions[] = array(
            'label' => "Some Label",
            'value' => "good product",
        );
        $observer->getProduct()->addCustomOption('additional_options', serialize($additionalOptions));
    }
}