iOSApplication.DefaultiPhoneScreen
From Xojo Documentation
Design Property (as iOSScreen)
This iOSApplication property can only be set in the Inspector and is not available in code.
Supported on Mobile (iOS).
Supported on Mobile (iOS).
Set to the default iOSScreen to use when the app is launched on an iPhone-sized device.
Notes
If you do not specify a DefaultiPhoneScreen, then the app will not run on iPhone-sized devices.
One or both of DefaultiPhoneScreen and DefaultiPadScreen must be specified.
By specifying separate screens for DefaultiPhoneScreen and DefaultiPadScreen, you can have completely different layouts for each type of device.