<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

use Magento\Quote\Api\CartRepositoryInterface;
use Magento\Quote\Model\QuoteFactory;
use Magento\Quote\Model\ResourceModel\Quote as QuoteResource;
use Magento\TestFramework\Helper\Bootstrap;

$objectManager = Bootstrap::getObjectManager();

/** @var QuoteFactory $quoteFactory */
$quoteFactory = $objectManager->get(QuoteFactory::class);
/** @var QuoteResource $quoteResource */
$quoteResource = $objectManager->get(QuoteResource::class);
/** @var CartRepositoryInterface $cartRepository */
$cartRepository = $objectManager->get(CartRepositoryInterface::class);


/** @var \Magento\GiftMessage\Model\Message $message */
$message = $objectManager->create(\Magento\GiftMessage\Model\Message::class);
$message->setSender('Romeo');
$message->setRecipient('Mercutio');
$message->setMessage('I thought all for the best.');
$message->save();

$quote = $quoteFactory->create();
$quoteResource->load($quote, 'test_quote', 'reserved_order_id');
$quote->setGiftMessageId($message->getId());
$cartRepository->save($quote);