save at core/components/<yournamespace>/controllers/<alias>.class.php and call it via /manager/?a=<alias>&namespace=<yournamespace> (make sure to add the namespace to you namespaces in MODX) – http://modxcookbook.com/customize-manager/cmps/cmp-made-easy.html

modParsedManagerController

<?php

class TestcmpParsedManagerController extends modParsedManagerController
{
    public function getPageTitle() {
        return 'Test CMP';
    }
    
    public function process(array $scriptProperties = [])
    {
        return '[[$testcmp]]';
    }
}