Automatically add product to cart on visit and auto-fill the cart in WooCommerce.

 /**

 * Automatically add product to cart on visit and auto-fill the cart in WooCommerce.

 */

function auto_add_to_cart_on_visit() {

    // Check if the user has already added the product to the cart.

    if (WC()->cart->is_empty()) {

        // Get the product ID you want to add to the cart automatically.

        $product_id = 123; // Replace with your product ID.

        // Get the product quantity to add to the cart.

        $quantity = 1; // Replace with the desired quantity.

        // Check if the product exists.

        if (wc_get_product($product_id)) {

            // Add the product to the cart.

            WC()->cart->add_to_cart($product_id, $quantity);

        }

    }

}

add_action('template_redirect', 'auto_add_to_cart_on_visit');