It is not necessary for an employer to install tracking applications to understand whether you are in place. Slack, Teams, and many other work apps automatically display your online status.
If you still want to leave your PC, you need to keep it awake and show that you're kind of there. In this case, special applications can help, but if their installation is undesirable, then this site.
All you need to do is go to the Stay Awake Club website and select the period during which you want to keep your computer from going to sleep. Available options: 30, 60, 90, 120 minutes and infinite.
Now you can safely go about your business, and a proud green dot will be lit in the working messenger.