m2-module-after-customer-login

\!h 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="customer_login">
    <observer name="custom_customer_login" instance="\Vendor\Module\Observer\CustomerLogin" />
  </event>
</config>

\!h CustomerLogin.php
<?php
namespace Vendor\Module\Observer;

use Magento\Framework\Event\ObserverInterface;

class CustomerLogin implements ObserverInterface
{
  protected $_url;
  protected $_session;
  protected $messageManager;
  protected $modelCart;
  protected $modelProductFactory;

  public function __construct(
      \Magento\Customer\Model\Session $session,
      \Magento\Framework\Message\ManagerInterface $messageManager,
      \Magento\Framework\UrlInterface $url,
      \Magento\Checkout\Model\Cart $modelCart,
      \Magento\Catalog\Model\ProductFactory $modelProductFactory
  ) {
      $this->_session = $session;
      $this->messageManager = $messageManager;
      $this->_url = $url;
      $this->modelCart = $modelCart;
      $this->modelProductFactory = $modelProductFactory;
  }

  public function execute(\Magento\Framework\Event\Observer $observer)
  {
    $productID = 1;
    $buyRequest = array(
      'product' => $productID,
      'qty' => '4'
    );
    $product = $this->modelProductFactory->create()->load($productID);

    $cart = $this->modelCart;
    $cart->addProduct($product, $buyRequest);
    $cart->getQuote()->setTotalsCollectedFlag(false);
    $cart->save();

    $CustomRedirectionUrl = $this->_url->getUrl('checkout/cart');
    $this->_session->setBeforeAuthUrl($CustomRedirectionUrl);
    $this->messageManager->addSuccess($product->getName());
    exit;
  }
}