Magento 2 : Set a custom Price after adding product

events.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_product_add_after">
        <observer name="set_custom_price" instance="MyNamespace\MyModule\Observer\SetCustomPriceAfterAddProduct" />
    </event>
</config>

SetCustomPriceAfterAddProduct.php

<?php
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <sony@menincode.com> <@>
 * @copyright Copyright (c) 2019 Menincode (http://www.menincode.com)
 */

namespace MyNamespace\MyModule\Observer;

use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;

class SetCustomPriceAfterAddProduct implements ObserverInterface
{
    /**
     * @event checkout_cart_product_add_after
     * @param Observer $observer
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function execute(Observer $observer)
    {
        /** @var \Magento\Quote\Model\Quote\Item $item */
        $item = $observer->getEvent()->getData('quote_item');
        $item = ($item->getParentItem() ? $item->getParentItem() : $item);
        $price = 100;
        $item->setOriginalCustomPrice($price);
        $item->getProduct()->setIsSuperMode(true);
    }
}