This hook enables you to add new WHMCS clients to your IEM contact list as they signup. Simply add (replacing the "XXXX") your IEM username, API Token, XML path & list ID# to the php file where indicated & upload the .php file to whmcs/includes/hook/ folder.
To find your List ID# - hover your mouse over the list 'Edit' link and your ID# will show up in the 'Status Bar' or click on the link & the # will show in the address bar.
Good luck