Apple pressure sensitive pen support is not yet available.
Currently spacedesk iOS Viewer only supports Sonar Pen https://www.sonarpen.com/
Full pen support is currently only implemented on Android.
Android is the only spacedesk Viewer supporting the whole spacedesk feature set.
Eventually all features available on Android will also be ported to iOS on Apple devices.
This includes Apple pen. However, no official timeline yet.