In general, opening a window in FullScreen mode is considered bad practice and is not recommended; hence, the Crossrider appAPI.openURL does not implement this feature. For more information, see Window.open and search for fullscreen in the page.
Having said that, you can try approximating the feature by opening a window with its width and height properties set to the corresponding window.screen properties.
[Disclaimer: I am a Crossrider employee]