Microsoft seeks Windows Insiders to test Cortana for iOS
If you are interested in receiving Cortana to test on your iPhone, you need to be a Windows Insider. Joining the program means that you will get updates for Windows at the same time that developers do. This allows you to let Microsoft know how it can improve the platform. To become a Windows Insider, go to https://insider.windows.com/ to sign up.
Once you are a Windows Insider, you need to fill out the survey found on the sourcelink. You'll be asked which phones you currently own, how often you employ Cortana on your Windows 10 PC, and what country you reside in. The Cortana for iOS update will at first be available only for the U.S. and China, but will eventually be offered in the same markets where you can find her on Windows 10.
Filling out the survey not only means that you might get access to the Cortana for iOS app before others, it also means that you might get other Cortana updates ahead of the public. These updates expand the personal assistant's functionality on the platform.
source: Survey via PocketLint