AFAIK, it is not possible in the sim. You have to take off, contact departure, then request it. I’ve tried tuning departure on the ground, referencing the correct frequencies outlined on the plates, but the sim doesn’t have the dialog options for that once tuned. I’ve honestly given up filing IFR with the default ATC, and instead follow my IFR plan while taking off asking for VFR and sometimes flight following. Too many headaches with default ATC.
IRL when on the ground at an uncontrolled airport you would absolutely be getting your IFR clearance prior to takeoff in the vast majority of cases, either by contacting departure via radio or phone, or sometimes Center depending on airport, but that’s not modeled here.