In order to enable DX12 support for the game you'll have to add the -dx12 or -d3d12 argument to the main executable, which you can find in the main game directory (RPG.exe).

If you are a Galaxy user and you still want to have the choice to select both DX11 and DX12, and use the Overlay as well, I added a new playtask to the config file: you'll just have to download this file and put it in the game directory.

If you want, you can choose to make the DX12 executable as the default one in the galaxy config menu (see attached image), the other one can be found under the "Additional executables" tab.

edit: updated file to patch 1.4.1

Start game with next keys:
Firmament.exe -d3d12 -nohmd