ext_localconf.php

if (TYPO3_MODE === 'FE') {
    $GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['cart']['showCartActionAfterCartWasLoaded'][] = \MyVendor\MyExtension\Hooks\FrontendUserHook::class . '->showCartActionAfterCartWasLoaded';
}

FrontendUserHook.php

Für neuere Versionen der Extension hat sich das NameSpace geändert:

\Extcode\Cart\Domain\Model\Order\Address- >\Extcode\Cart\Domain\Model\Order\BillingAddress 


<?php


namespace MyVendor\MyExtension\Hooks;

use DmitryDulepov\Realurl\Configuration\ConfigurationReader;

/**
 * FrontendUserHook
 *
 * @author Daniel Lorenz <[email protected]>
 */
class FrontendUserHook
{
    /**
     * @param array &$parameters
     */
    public function showCartActionAfterCartWasLoaded(&$parameters, $refObj)
    {

        $billingAddress = $parameters['billingAddress'];

        $request = $parameters['request'];

        if ($billingAddress instanceof \Extcode\Cart\Domain\Model\Order\BillingAddress) {
            return;
        }

        if ($request && $request->getOriginalRequest() && $request->getOriginalRequest()->getArguments()) {
            return;
        }

        $feUserUid = (int)$GLOBALS['TSFE']->fe_user->user['uid'];

        $objectManager = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(
            \TYPO3\CMS\Extbase\Object\ObjectManager::class
        );
        $frontendUserRepository = $objectManager->get(
            \TYPO3\CMS\Extbase\Domain\Repository\FrontendUserRepository::class
        );
        $frontenUser = $frontendUserRepository->findByUid($feUserUid);

        if ($frontenUser instanceof \TYPO3\CMS\Extbase\Domain\Model\FrontendUser) {
            $billingAddress = $objectManager->get(
                \Extcode\Cart\Domain\Model\Order\BillingAddress::class
            );

            $billingAddress->setEmail($frontenUser->getEmail());
            $billingAddress->setTitle($frontenUser->getTitle());
            $billingAddress->setFirstName($frontenUser->getFirstName());
            $billingAddress->setLastName($frontenUser->getLastName());
            $billingAddress->setCompany($frontenUser->getCompany());
            $billingAddress->setStreet($frontenUser->getAddress());
            $billingAddress->setZip($frontenUser->getZip());
            $billingAddress->setCity($frontenUser->getCity());
        }

        $parameters['billingAddress'] = $billingAddress;
    }
}

Here is a version for TYPO 10.4:
https://gist.github.com/martinlipp/83a145b1b3faa0ed6f6f5d74462cbc37