For Joomla 3.x and Joomla 4, there is a generally accepted way to connect third-party PHP libraries. Link to official Joomla documentation. Starting with Joomla 3.1.2, the Joomla kernel class loader supports PSR-0 autoloading of namespaces. In order for the kernel loader to know about the namespace of the library, you need to use a system plugin that triggers the onAfterInitialize event. This plugin is bundled with the WT Amocrm library. It is also used to store amoCRM credentials in Joomla,

Connecting the library to your Joomla extension

use Webtolk\Amocrm\Amocrm;
 
$amocrm = new Amocrm();
$result_amo_crm = $amocrm->getAccountInfo();