Welcome to the forum! You do not need to install anything onto the NVMe before StartOS. If you have access to another computer, you can flash the installer onto your USB thumb drive there, and then boot from it on the UN1290. In the other video, I just flashed it from the Minisforum since it already had Windows installed and was already set up in front of the camera.
Just a heads up about the UN1290 model – you will need a keyboard, mouse, and monitor temporarily, because it won’t boot after you finish the installation. I posted instructions for how to fix that problem over on another thread here. Once you apply the fix, then you won’t need the keyborad, mouse, and monitor anymore.