Activity ID? Possibly do you mean activity class? If so then click the tiny "information" button out to the side of it and there is a pop up that gives you how Garmin rates your activity class. This may be different for different sites.
If by activity ID you mean device ID or unit ID then that can be seen with Garmin Express and other Garmin programs or just go in the "about" page of your device setup.
Might be a language miscommunication here as we don't see the same titles for things on the Garmin page here as you may in your country.
