When you click on the "share screen" icon in the upper right of Sococo, a selection window will appear. If you do not see a thumbnail of the program or document you want to share, or if what you want to share is showing as a black thumbnail, check to see if the program or document you want to share is open in the background (not minimized to your taskbar or dock). Once the document is open in the background, switch back to Sococo by choosing it from your taskbar or dock, then click on the "share screen" icon again. You should now see a thumbnail available of the window you wish to share. If it continues to show as black, there may be a security setting related to that program that is blocking its ability to be shared. A workaround is to share your "entire screen" (or monitor, if you see Screen 1, Screen 2, etc.) and then maximize the program or document you wish to share on your monitor.
REMINDER: If you choose to share your entire screen (or monitor), viewers will see all activity on that particular monitor, just as if they were sitting in front of that monitor.