There isn't a tick box as such, in the AvastUI > Protection > Antivirus > Other Scans section there will be various scans, default or custom, etc. If you look at the top right of each small window for the scan you should see a Clock, guess what that is for, very cryptic I know.
That said, many people are having issues in even running a scheduled scan and as far as I'm aware avast are trying to resolve that problem.