Need help with Mailster autoresponders.

We set up an Mailster autoresponder that triggers when WooCommerce order status is completed. Problem is, it’s sending the email to all the subscribers in the list instead of the one who triggered the autoresponder. We have this code to trigger the custom hook:

function trigger_autoresponder($order_id) {
$subscriber_id = mailster_get_current_user_id();
do_action( ‘order_completed’, $subscriber_id );

// add the action
add_action( ‘woocommerce_order_status_completed’, ‘trigger_autoresponder’, 10, 1 );

but it’s still sending to all the subscribers on the list. Please check and help.