To share your screen just hit the Share button, then in the new modal that appears, you can choose if you want to share your full screen or a single tab, once you choose just to hit the share button and your screen will appear!
Screen sharing
Check out the helpdesk article below and let us know if you still need support!

How can I share my screen in Markee?
If you are running macOS Catalina — version 10.15 or later — you must grant access to the new Screen Recording Permission in order to share your screen during a video call. Here you can found how.
We are so sorry you are having an issue. Please don’t hesitate to contact us at support@markee.io if your issues persist.