In the LNM map, point your mouse to the green parking stand at your departure airport and right click, In the context menu, look for the Set (parking location) at Airport (airport name) as Departure option (the location and airport name will be filled in for the parking stand you select). A yellow selection ring will surround your starting parking spot on the map.
Then export your PLN file (File|Export Flight Plan as MSFS 2020 PLN…) and the parking location will be imbedded in the PLN file. The sim will populate this parking location in the world map when you load your PLN file and you will start cold and dark at your parking spot when the sim finishes loading.
If I forget the parking location step in LNM, I can still add the parking location when loading into the world map, before starting the sim.
I use Pilot2ATC and SimBrief for my CJ4 flights. I let Pilot2ATC manage my runway selection and taxi instructions. My workflow is to plan the flight in SimBrief. Click the Generate Flight button to generate the briefing package. Go to Pilot2ATC, select FP Import from the right-side menu, check that your SimBrief PilotID is entered, then click the Import SimBrief button. I have Import Procs checked on the optional items to the right of the Import button.
The flight plan will populate into the main Pilot2ATC window. Click the Export button at the bottom, check that you have set the MSFS 2020.pln location. At the bottom of the Export window, click the option boxes you want, then click the Export button. The PLN file is ready to open in LNM to add your starting parking location and any other changes you wish to make, then export the PLN file for MSFS.
If not using LNM, load the exported PLN into the MSFS world map and make changes on the world map screen, primarily choosing your parking location.
I am using the AAU1 beta for the CJ4 and it no longer allows importing plans from the IDX button (no longer an active button). If you are using the AAU1 beta or after the AAU1 releases, this will be the procedure to enter flight plans.