Email Presend: Intercept invoices for all clients that use automated billings

[txrodriguez]

Asking for help, not a developer and need to figure out how I can stop invoice reminders being sent to all clients that have signed up for automated billing.  I was told to use an email presend I would appricate any help to help fill in the code.  https://developers.whmcs.com/hooks-reference/everything-else/#emailpresend

Also which folder would I upload this too.  Thanks in advance. 

 

<?php

add_hook('EmailPreSend', 1, function($vars) {
    $merge_fields = [];
    $merge_fields['my_custom_var'] = "My Custom Var";
    $merge_fields['my_custom_var2'] = "My Custom Var2";
    if ($vars['messagename'] =='My Message Name' && $vars['relid'] == 2) {
        //Stop the email from sending a specific message and related id.
        $merge_fields['abortsend'] = true;
    }
    return $merge_fields;
});