Through trial and error, I found the following works on iOS (the equivalent of above on Android)
// android: top, bottom, left, right
InAppBrowser.EdgeInsets insets = new InAppBrowser.EdgeInsets(0, 0, 100, 100);
options.insets = insets;
// ios: left, bottom, top, right
InAppBrowser.EdgeInsets insets = new InAppBrowser.EdgeInsets(100, 0, 0, 100);
options.insets = insets;
I couldn’t get the original app to show behind the browser in iOS like it does on Android.