you must edit the f1graphic.cfg file and set the following line to 1
0 ; [RELEASE] Run in a window
Then you adjust the resolution size for that window so it doesn't take your complete screen. What resolution you choose is dependant what screen you have. The bigger the better of course
You set the resolution also in that file in the following lines:
1920 ; [RELEASE] Game 3D Horizontal Resolution
1080 ; [RELEASE] Game 3D Vertical Resolution
thats mine resolution on a 23 inch monitor which take of course the full screen size