Mage2\Wishlist\Controllers\WishlistController::add PHP Method

    public function add($slug)
    {
        $id = $this->_getProductIdBySlug($slug);
        Wishlist::create(['user_id' => Auth::user()->id, 'website_id' => $this->websiteId, 'product_id' => $id]);
        return redirect()->back()->with('notificationText', "Product Added into your Wishlist Successfully!!");
    }