New guy here, but flight simmer since version 1. I've hard coded the tail number in for the 172 glass version so it comes up everytime.
The location on my system is here:
\Program Files (x86)\Microsoft Games\FS2020\Official\Steam\asobo-aircraft-c172sp-as1000\SimObjects\Airplanes\Asobo_C172sp_AS1000
I changed this line, but there is another line for "atc flight number" which I'm sure could be coded as well:
atc_id = "123CL" ; tail number