Ok, so the Internet is crawling with plenty of howtos for configuring different/many operating systems during boot with GRUB2. But I haven’t found the single one, how to make it depended on simple key press – not have to choose from GRUB menu.
So, in my case, I want to have boot up Linux/Debian all the time – as the default system, but from time to time, use Windows for Fusion 360 (or for gaming). Of course I can choose different OS from GRUB menu – like everyone else, but it would be much nicer and faster to simply press „shift” during boot and see Windows.
And yes, we can do it 🙂