How to use external libraries in a plugin

This is probably a stupid question, but here it is anyway; I’ve added a library with composer to my plugin, but I don’t know how to instantiate my class: none of these require statements work:

require_once(__DIR__ . '/vendor/autoload.php');
            //require_once (__DIR__ . '/vendor/campaignmonitor/createsend-php/csrest_subscribers.php');
            //$subscribers = require_once(__DIR__ . '/classes/createsend/csrest_subscribers.php');
            $LIST_id = $this->config->get('plugins.campaignmonitor.listid');
            $API_key = $this->config->get('plugins.campaignmonitor.apikey');
            $this->createsend = new CS_REST_Subscribers($LIST_id, $API_key);

I get: PHP Fatal error: Class ‘Grav\Plugin\CS_REST_Subscribers’ not found
So it’s looking in the plugin namespace, instead of looking for the composer library.
How should I do this?

Here’s a very simple example: