cx-newsletter/endpoints/3-settings_check_endpoint.php

<?php

namespace cx_newsletter;
use \wp_rest_request;

class settings_check_endpoint 
extends rest_endpoint
implements rest_endpoint_interface {
    use rest_endpoint_with_apikey;

    public function get_method() : string {
        return 'GET';
    }

    public function get_route() : string {
        return '/validate/';
    }

    public function action(wp_rest_request $request) : array {
        if (!$this->check_request_apikey($request)) {
            return [ 'Validation' => false ];
        } 

        return [ 'Validation' => true ];
    }
}