Using composer without framework

Question

I have php script which is very simple, so I don't use any framework like symfony

My folder structure is here below

myapp
  -vendor
  -composer.jspn
  -script
    -myapp.php

So I want to use library in myapp.php

use PhpOffice\PhpSpreadsheet\Spreadsheet;
$spreadSheet = new Spreadsheet;

It says Uncaught Error: Class 'PhpOffice\PhpSpreadsheet\Spreadsheet' not found

What is the best practice for this case??