Hi @toem,
Tha spacedesk display connected is in desktop extension mode, you can verify it by opening the Windows Display settings and you will see an extension screen beside your main monitor screen.
If you want to duplicate/mirror your main monitor screen to your spacedesk viewer app, just select the “Duplicate” option in the display settings or use Display switch (Windows logo + P hotkey).
For assistance, please check the chapter – Configuration and Settings -> Windows Driver (Primary Macine) in the user manual https://forum.spacedesk.netuser-manual/#item_469.