Screen transmission via video telephony is (currently) only supported via the browser on the computer and on Android devices. This means that the screen cannot be shared on Apple devices such as an iPad.
To share your screen, you will see the following icon in the lower left corner:
Click on it to share your screen with the participants in the conference.
If this does not work right away, please check whether both the computer and the browser are allowed to share the screen with Sdui Meet (call.sdui.de).