Add custom Placeholder / Shortcode to Mailbuilder

functions.php

<?php
add_filter( 'haet_mail_placeholder_menu', 'add_mailbuilder_placeholder' );
add_filter( 'haet_mail_order_placeholders', 'populate_mailbuilder_placeholder', 10, 3 );

function add_mailbuilder_placeholder( $placeholder_menu ){
    if( is_array( $placeholder_menu ) ){
        $placeholder_menu[] = array(
                'text'      => 'My Placholder Name',
                'tooltip'   => '[MY_PLACEHOLDER]',
            );
    }

    return $placeholder_menu;
}


function populate_mailbuilder_placeholder( $order, $wc_order, $settings ){
    $order['my_placeholder'] = 'your value';
    return $order;
}