It's an extra cost. $100 to $200. You can't buy it, generally, except through a volume licensing partner. You may need to have a tenant ID depending on exactly how you're getting it, too. Alternately, you need to have a Visual Studio subscription which is $3k/yr. Oh, and you can't upgrade to LTSC from Pro. You have to do a fresh install. And IoT LTSC is even worse.