Yeah it's not the most intuitive of websites I'll admit. If you haven't made an account, make one first just so you're logged in. Then you should be on
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Then it should show you a list of options. Choose record. Then at the bottom of the screen should be an option to screen share. You have the option of a tab, window or whole screen. Make sure the audio is being shared too, otherwise it'll just record muted. Once you get it sorted and get used to using it, it's a really useful tool to have