As far as I'm aware there is no shortcut, it is a somewhat special scan that has no settings. The only access is through the avastUI > Scan > Smart Scan.
I can't see why they/avast isolated it from the other Scans in the Drop Down list, even if they didn't allow changes to the settings, just the ability to schedule it. That said the Smart Scan is a bit more interactive because of the different things it is doing other than just a virus scan, this may be why it isn't allowed.