to start it in windowed fullsreen mode simply open the config file in "%APPDATA%/Secrets of Grindea" with the editor of your choice and change
"Fullscreen [0 = windowed, 1 = fullscreen, 2 = windowed fullscreen]
0"
to
"Fullscreen [0 = windowed, 1 = fullscreen, 2 = windowed fullscreen]
2"