If you already bought a Steganos Online Shield Premium license for Windows or Android, you can of course use it in the iOS app too.
You need your serial number (You have received your serial number by e-mail, can find it on the retail box or inside the Android app, depending on how you bought the license).
Install the iOS app if you haven't already. In the app's menu under "Settings" you can find an ID displayed at the bottom (you can easily copy it by holding it pressed). Now click on this link:
On the registration site, enter the app ID and your serial number, then click on "Activate".
After restarting the app it should now be fully activated.