add_action( 'woocommerce_product_options_general_product_data', 'cwpai_woo_add_custom_general_fields' );
function cwpai_woo_add_custom_general_fields() {
    global $woocommerce, $post;
    echo '<div class="options_group">';
    woocommerce_wp_text_input(
        array(
            'id' => '_cwpai_woo_redirect_url',
            'label' => __( 'Redirect URL', 'woocommerce' ),
            'placeholder' => 'http://',
            'desc_tip' => 'true',
            'description' => __( 'Enter the URL the customer will be redirected to after purchasing this product.', 'woocommerce' )
        )
    );
    echo '</div>';
}
add_action( 'woocommerce_process_product_meta', 'cwpai_woo_add_custom_general_fields_save' );
function cwpai_woo_add_custom_general_fields_save( $post_id ){
    $woocommerce_text_field = $_POST['_cwpai_woo_redirect_url'];
    if( !empty( $woocommerce_text_field ) )
        update_post_meta( $post_id, '_cwpai_woo_redirect_url', esc_attr( $woocommerce_text_field ) );
}
add_action( 'woocommerce_thankyou', 'cwpai_woo_redirectcustom');
function cwpai_woo_redirectcustom( $order_id ){
    $order = new WC_Order( $order_id );
    $items = $order->get_items();
    foreach ( $items as $item ) {
        $product_name = $item['name'];
        $product_id = $item['product_id'];
        $product_variation_id = $item['variation_id'];
    }
    $redirect = get_post_meta( $product_id, '_cwpai_woo_redirect_url', true );
    if ( ! empty( $redirect ) ) {
        wp_redirect( $redirect );
        exit;
    }
}