While Richard_0101's answer should work for a single file, it would not do the scan for which you seem to be asking.
So, it seems to me that Bob3160's answer is what you need to know. As bob says, "Since anything added or changed on your computer is checked for an infection by avast's on demand scanner before you ever get a chance to use the program ......" there should be no reason to need an extra scan at the computer's start-up no matter what time of day you do this. Everything new or changed would have already been scanned a prior time to the computer being shut down.
As an example, my home desk top computer is down for days at a time when I am on trips. I see no need for scanning the computer upon my return home because with the computer down during those days, nothing has been added nor changed.
Two clarifications are needed. Avast does a root kit scan at each start-up. A boot time scan can also be set but it is very long and should only be used when a problem is evident.
I hope this helps you,