The manual has a detailed explanation of how to do this here: https://www.avplan-efb.com/manuals/avplan-efb-ios-manual/9.x/en/topic/creating-editing-an-aircraft-instance.

Here is something to note:

The Callsign of your aircraft is not the registration- just the callsign. So in Australia and NZ that would typically be just the last three letters of the registration. Please do not put VH-AAA or ZK-BBB into the callsign field. Doing so will prevent succesful filing of flight plans, and whether or not you file flight plans, it may also cause AvPlan to draw multiple traffic icons for a single aircraft that also has an ADSB-OUT transponder.

Leaving the Callsign Blank is not a good idea - aircraft without a callsign cannot be used for a flight plan. And all flight plans need an aircraft so that AvPlan knows what performance data (especially speeds and fuel burn rates) to use.