Yes it’s possible. You just have to attach your physical monitor first so that you can install the spacedesk Driver on your primary machine.
After spacedesk driver installation, you can detach now your primary monitor and just try to connect from your secondary device via spacedesk viewer app.
Please make sure that both primary machine and secondary device are connected in a local area network.
You can also check this thread: https://www.spacedesk.net/forums/topic/headless-pc/