There are a couple ways to do a screen share from a secondary monitor.
One is to simply move Biba over to your secondary monitor by clicking on the top bar of the call window and scooting it over, then share your screen like you normally would by clicking on Screen from the call controls, then selecting either Share Desktop to share your entire screen, or Share Window to share a specific window.
Another way is to share your screen on your primary monitor by clicking on Screen from the call controls, then selecting Share Desktop.
While sharing your screen, click Screen again and choose either Select Desktop or Select Window. Then move your mouse over to the secondary monitor and click on what you wish to share.