Laravel: Can't use the autoload function - laravel

Question

I have a question about autoloading in Laravel.
I am trying to use an API to translate a web site. I typed in the code given by the provider in the controller and I got the following error message.
require(vendor\autoload.php): failed to open stream: No such file or directory
This is the code.
require 'vendor\autoload.php'; <=★Here★
session_start();
define('URL', '');
define('KEY', '');
define('SECRET', '');
define('NAME', '');
$api_name = '';
$api_param = '';
$provider = new \League\OAuth2\Client\Provider\GenericProvider(
[
'clientId' => KEY,
'clientSecret' => SECRET,
'redirectUri' => '',
'urlAuthorize' => '',
'urlAccessToken' => URL . '/oauth2/token.php',
'urlResourceOwnerDetails' => '',
],
);
try {
// Try to get an access token using the authorization code grant.
$accessToken = $provider->getAccessToken('client_credentials');
// The provider provides a way to get an authenticated API request for
// the service, using the access token; it returns an object conforming
// to Psr\Http\Message\RequestInterface.
$params = array(
'access_token' => $accessToken->getToken(),
'key' => KEY,
'api_name' => $api_name,
'api_param' => $api_param,
'name' => NAME,
'type' => 'xml',
'xxx' => 'xxx',
);
$request = $provider->getAuthenticatedRequest(
'POST',
URL . '/api/?' . http_build_query($params),
$accessToken,
);
$response = $provider->getResponse($request);
$data = $response->getBody()->getContents();
print_r($data);
} catch (\League\OAuth2\Client\Provider\Exception\IdentityProviderException $e) {
// Failed to get the access token or user details.
exit($e->getMessage());
}
I checked the file pointed out in the error message and actually autoload.php file was in the vendor folder. I did some research and I found some web sites saying that I should add a directory in composer.json but I wasn't so sure what I should do.
I appreciate if you could tell me what I'm doing wrong or what I should do. Any help would be appreciated as I have tried multiple methods with no success.
Thank you in advance.