Based on the tutorial video, you are connecting via HTML5 Viewer which is also running on your primary machine, right?
I assume when you connect the HTML5 viewer on your machine, it’s in mirror mode. But you want it in extend mode, right?
Then you can easily switch to extend mode, just open the Windows Display Settings and switch the mirrored displays to extend.
For assistance, please check Configuration and Settings -> Windows Driver (Primary Machine) chapter https://forum.spacedesk.netuser-manual/#item_469.