I wanted to install Windows 10 on my new FreeDos notebook, but I don't have a USB stick at hand. Is it possible to use an SD card as a boot stick?
Yes, provided it has enough space. Should be the same procedure as with a USB stick.
Your SD card reader must also be a boot option in the BIOS.
The prerequisite is that your BIOS allows entry in the boot sequence and you have made the entry.