To enter enter the transponder hex code for your aircraft into an ICAO flight plan, please do the following:
1. Go to MORE > AIRCRAFT and open the Aircraft Profile you will be filing with by tapping on the (i) button on that aircraft line.
2. From the pop up, select OTHER INFORMATION and then CODE. Enter the HEX code for that aircraft.
3. Tap the BACK BUTTON (Black Arrow that has the word "Aircraft" in white font) to store the results.
4. Tap DONE.
Now, when you file a flight plan using that aircraft profile, the Hex code will be included.