At the moment we do not support entering private flights. We will be adding flight entry without flight numbers soon.
What to do if you travel on a private flight
For now, find a commercial flight on the same route, or departing and arriving in the same time zones, add that and then adjust the times to fit your private flight afterwards. This will generate the right jet lag plan for you.
You can adjust the times by tapping the flight in the flight list after you submitted the flight(s).