Add billable_invoice email template to Email Templates Admin Editor

billable-invoice-pmproet.php

<?php
/**
 * Add the billable_invoice email template to the Email Templates Admin Editor.
 * Follow this guide to add custom code to your WordPress site - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function my_add_email_template_to_pmproet_add_on( $template ) {
    $template['billable_invoice'] = array(
        'subject' => "Your latest invoice",
        'description' => 'Billable Invoice'
    );

    return $template;
}
add_filter( 'pmproet_templates', 'my_add_email_template_to_pmproet_add_on' );