Locate C:\Program Files (x86)\Steam\steamapps\common\Hakoniwa Explorer Plus\Save\option.dat , then open it in Notepad and you'll be met with a string of numbers that are concatenated by commas and | symbols.

This is the string within my option.dat: 0,0,16,16,4,0,0,0,37|38|39|40|65|83|90|88,11|13|12|14,1,0,0,

The third option from the right (currently 1) is the key here:
1 = full screen
0 = windowed

Changing that one number allowed me to go fullscreen whereas it was "window" in red text before.

NOTE: On my GPD Win, my scaling is set to 130% for the sake of readability, and it made Hakoniwa display a lot of information out of bounds. I fixed this by:
- Right click the game.exe (C:\Program Files (x86)\Steam\steamapps\common\Hakoniwa Explorer Plus\)
- Properties, then compatibility tab
- Change High DPI Settings
- Check the "Override high DPI scaling behavior." box
- In the drop down menu within that same section, allow Scaling to be performed by Application.

Not sure if any of that scaling config will be necessary for your specific case, but it has the game fullscreen and displaying perfectly on my GPD Win.