As a Windows Insider, you already know that Cortana is your truly personal digital assistant, there to help you get things done. By learning more about you over time, Cortana becomes more useful every day. Cortana helps you be more productive by completing basic tasks like sending emails, scheduling events, and using the power of Bing to quickly search your devices, the cloud, or the web. Cortana’s also the best at reminders, delivering them at the right time and place so you forget less and can do more.

Your life is not limited to one device, and neither is your digital assistant. Cortana is with you on your Windows 10 PC, tablet, and phone—helping you whenever and wherever you need it. But we also know that there's a choice of mobile devices out there, so we want help from our Windows Insiders to make sure she's a great personal assistant on iOS too.

Cortana's deep integration with Windows means that’s where she can really shine. So, while there are some features you'll use on Windows that we don't (yet) support on iOS phones (like saying "Hey Cortana" to wake her up), we’ve focused on making Cortana on iOS the perfect companion to your Windows experience.

In the spirit of the Windows Insider Program, we’re looking for a limited number of people to get their hands on an early version of the app. It’s important to keep in mind that this is the first public release of the Cortana for iOS beta. And that for now, Cortana for iOS is only available in the U.S. and China. Over the coming months, we’ll continue to deliver frequent updates to the app to expand the features and functionality.

If you're interested in getting early access to the beta version in the U.S., please take this short survey. We'll be limiting access to begin with, and will start sending out download links over email to bring users on-board in the coming weeks.

Source: Introducing Cortana for iOS - Microsoft Community