Access WP Job Manager Class from custom plugin

Hello,

Thanks for great plugin.

I want to access a class of WP Manager(e.g. WP_Job_Manager_Ajax) from custom plugin, but it throws error

if(!class_exists(‘WP_Job_Manager_Ajax’)) {

require_once ROOTDIR. ‘../wp-job-manager/includes/class-wp-job-manager-ajax.php’; // <== Cannot redeclare class WP_Job_Manager_Ajax
$wpjob = new WP_Job_Manager_Ajax(); //Class ‘WP_Job_Manager_Ajax’ not found
}

Could you please help me to find out how can I achieve it.

Thanks for your help
Regards
Venom