To add a mask, click the Manage Camera icon on the toolbar. Select the camera and click the Edit Camera button. Go to the Mask Area tab and select the Enable Masking checkbox. Adjust the shape of the mask according to your requirement by clicking on the box and dragging it. Add additional masks for the areas you don't want to monitor. Click the OK button to save your changes.

Once you put a mask on the camera preview, start motion detection monitoring. Notice how motion behind the masked region will not get detected. This will reduce the possibility of unwanted triggering of alerts.