Need example of a "Notify URL"

Using the Enhanced PayPal shortcode. Gave it FIVE stars.

1) Please give us an example of what a “notifyurl =” should look like.

2) Is it necessary for us to have a ” notifyurl = ” ?

Thanks in advance.

http://wordpress.org/extend/plugins/enhanced-paypal-shortcodes/