theres a plugin called GiveMiiYoutube on hb store idk if u need anything else tho

If the app isn't already on your Wii U, you don't. It's not on the update servers at all anymore, so you can't redownload it from the eShop. If you do have it, you can use the GiveMiiYouTube Aroma plugin from the HB App Store to get it working again!