Hi. :)
If you press Q to quit (or Alt-Q to exit to the main menu) and then press Enter at the prompt, it will save your game but also quit to the desktop or main menu, which probably is not ideal.
I tried a bunch of key combinations just now but couldn't find one for just bringing up the Save prompt by itself. Hopefully I just missed it.
If you download the mod
NRaas Saver, it will automatically bring up the Save prompt every few minutes (30 minutes by default), then you'd just have to press Enter from that prompt to save your game.