if I would like to use the screenshare option but I would like to decide which part of my screen is shared (rather than all of it); how can this be done?
Apparently I cannot just click on the blue box.
Hi, what blue box do you mean?
You have option to select specific application and share only its windows.
You can not share arbitrary part of a screen.
try to use some extensions. There are a lot of tools that can help you to choose the desired part of a screenshot.