To install PHPMailer and make it available across all domains/virtual hosts on your GoPanel server
$ /go/vendor phpmailer/phpmailer

Verification Test:
Create a PHP file in any domain:

<?php
require "vendor/autoload.php";

$mail = new PHPMailer\PHPMailer\PHPMailer();
if (class_exists('PHPMailer\PHPMailer\PHPMailer')) {
    echo "PHPMailer is working!";
} else {
    echo "PHPMailer not found";
}
?>