For me it was very weird. The files have a prefix attached to them, basically a reference to a folder. I had to go to "Associated Packages & DLC", then you click on the build (probably colored in yellow or red). Then where it says "Depots Included", you want to click on the corresponding depot, and it should bring up the "Depot Manifest (AppID)" Page where you can see all the associated files. Just copy the path of the .exe file exactly, and paste it in your launch option. That is what fixed this problem for me! (Very proud of myself for figuring this out lol)