composite-back-in-stock.php

add_action('woocommerce_after_add_to_cart_form', 'cr_custom_bis_button_composites_only', 9);
function cr_custom_bis_button_composites_only() {
    if (!function_exists('WC_BIS')) return;

    global $product;

    if (!$product || !$product->is_type('composite') || $product->is_in_stock()) return;

    // Output form matching plugin's expected structure
    ?>
    <div id="wc_bis_product_form" data-bis-product-id="<?php echo esc_attr($product->get_id()); ?>">
        <?php if (! is_user_logged_in()) : ?>
            <input type="text" id="wc_bis_email" name="wc_bis_email" class="input-text" placeholder="<?php esc_attr_e('Enter your e-mail', 'woocommerce-back-in-stock-notifications'); ?>" />
        <?php endif; ?>
        <button class="button" type="button" id="wc_bis_send_form" name="wc_bis_send_form">
            <?php esc_html_e('Notify me', 'woocommerce-back-in-stock-notifications'); ?>
        </button>
    </div>
<?php
}

Running Woo back in stock notifications with composite products. (not supported).

This basically adds the button for you (in my case, I only wanted it to run on Composite products)