If you see that the Share option is not active in the "Choose what to share..." window, you need to click the "Entire screen" area under the Entore Screen tab.
Thus, your seletion activates the Share option:
If you see that the Share option is not active in the "Choose what to share..." window, you need to click the "Entire screen" area under the Entore Screen tab.
Thus, your seletion activates the Share option: