Environment::getSyntheticFrontendRequest() 

public function getSyntheticFrontendRequest( $pageUid = null )
{
	if (isset($this->SYNTHETIC_FE_REQUEST)) {
		return $this->SYNTHETIC_FE_REQUEST;
	}
	// Resolve site + language + a page id (use site root as default)
	$pageUid  = $pageUid ?: (int) (\nn\t3::Page()->getSiteRoot()['uid'] ?? 0);
	$site	 = GeneralUtility::makeInstance(SiteFinder::class)->getSiteByPageId($pageUid);
	$langId   = $this->getLanguage();
	$language = $site->getLanguageById($langId);
	// Build base URI (fallback if site base is empty)
	$base = (string)$site->getBase();
	if ($base === '' || $base === '/') {
		$base = $this->getBaseURL();
	}
	$uri = new Uri($base);
	// Start request (URI first, then method in v13)
	$queryParams = ['id' => $pageUid]; // make routing happy
	$request = (new ServerRequest($uri, 'GET'))
		->withAttribute('site', $site)
		->withAttribute('language', $language)
		->withQueryParams($queryParams);
	// normalizedParams (what older code used to get from getIndpEnv)
	$serverParams = [
		'HTTP_HOST'	  => $uri->getHost() . ($uri->getPort() ? ':' . $uri->getPort() : ''),
		'REQUEST_METHOD' => 'GET',
		'REQUEST_URI'	=> $uri->getPath() === '' ? '/' : $uri->getPath(),
		'QUERY_STRING'   => http_build_query($queryParams),
		'HTTPS'		  => $uri->getScheme() === 'https' ? 'on' : 'off',
		'SERVER_PORT'	=> $uri->getPort() ?: ($uri->getScheme() === 'https' ? 443 : 80),
	];
	$normalized = NormalizedParams::createFromServerParams($serverParams);
	$request = $request->withAttribute('normalizedParams', $normalized);
	// routing (PageArguments) — minimal but sufficient
	$pageType = 0;
	$routing  = new PageArguments($pageUid, $pageType, $queryParams);
	$request  = $request->withAttribute('routing', $routing);
	$request = $request->withAttribute(
		'applicationType',
		SystemEnvironmentBuilder::REQUESTTYPE_FE
	);
	// ensure full TypoScript even in "cached" context
	$request = \nn\t3::Tsfe()->softDisableCache($request);
	// Tiny handler to capture the mutated request
	$captured = null;
	$handler = new class($captured) implements RequestHandlerInterface {
		public ?\Psr\Http\Message\ServerRequestInterface $captured;
		public function __construct(&$_c) { $this->captured = &$_c; }
		public function handle(\Psr\Http\Message\ServerRequestInterface $request): \Psr\Http\Message\ResponseInterface {
			$this->captured = $request;
			return new HtmlResponse('');
		}
	};
	// needed to prevent cross-contamination when in backend-context
	$oldRequest = $GLOBALS['TYPO3_REQUEST'] ?? null;
	// initialize FE controller (sets frontend.controller / $GLOBALS['TSFE'])
	GeneralUtility::makeInstance(TypoScriptFrontendInitialization::class)->process($request, $handler);
	$request = $handler->captured ?? $request;
	// prepare TypoScript (attaches frontend.typoscript with FULL setup now)
	GeneralUtility::makeInstance(PrepareTypoScriptFrontendRendering::class)->process($request, $handler);
	$request = $handler->captured ?? $request;
	$this->SYNTHETIC_FE_REQUEST = $request;
	// reset to global request before capturing
	$GLOBALS['TYPO3_REQUEST'] = $oldRequest;
	return $request;
}