I'm not 100% sure what the answer to this is. I know that people using a Windows 10 PC with an Android phone are able to make it work. But I think it'd be pretty easy to setup. Install Cortana on your iPhone, and get it setup. Once you have Cortana setup on your iPhone, and you have Cortana setup on your Windows 10 PC, go ahead and type "Text _____" into Cortana on your desktop. Let us know how it goes!