How to call UMUM class in CI4?

My fpdf (version 1.85) folder is in App\ThirdParty.
I added the following to autoload.php in config:

PHP Code:

public $classmap = [
        "FPDF"          => APPPATH . "ThirdParty/fpdf/fpdf.php",
    ]; 


My test method looks like this:

PHP Code:

public function fpdfTest()
{
$this->pdf = new FPDF('P', 'mm', 'A4');
$this->pdf->AddPage();
$this->pdf->SetTextColor(42,80,125);
$this->pdf->SetFont('Helvetica','B',18);
$this->pdf->Text(15,12.5,"test");
$this->pdf->SetFont('Helvetica','B',14);
$this->response->setHeader('Content-Type','application/pdf');
$this->pdf->Output('D', 'Invoice_test.pdf');
} 


A PDF is created and automatically saved/opened (depending on browser settings)