Select the "Video" icon from the top toolbar. Each camera connected to your computer will appear as a button in the preview area. The camera being used will appear with a check mark over it. Deselect the camera buttons that include a checkmark: 

Select the "Screens and Apps" icon from the top toolbar: 

The “Share your screen” menu will appear. To select your screen, select “Your Entire Screen,” select the screen you want to present, and then select “Share.”  If you want to share your screen’s audio, select the checkbox next to “Share audio.”  Note: in this example image, the user has two screens: a laptop screen and an additional monitor. If you only have one screen, or share the screen you are using Panopto Express on, it will create an endless, mirrored effect until you minimize the Express recorder. Your screen will still be recorded in this case.