Can I use the command AT in Windows 8.1?

No you cannot. AT command has been deprecated(used no more).
Instead you should use schtasks.exe.

How do you use it?

Open up a command prompt window (AKA DOS window) preferably with administrator privileges.

Just type as shown and you should get the help menu.

C:\Windows\system32>schtasks /?
SCHTASKS /parameter [arguments]
Description:
    Enables an administrator to create, delete, query, change, run and
    end scheduled tasks on a local or remote system.
Parameter List:
    /Create         Creates a new scheduled task.
    /Delete         Deletes the scheduled task(s).
    /Query          Displays all scheduled tasks.
    /Change         Changes the properties of scheduled task.
    /Run            Runs the scheduled task on demand.
    /End            Stops the currently running scheduled task.
    /ShowSid        Shows the security identifier corresponding to a scheduled t
ask name.
    /?              Displays this help message.
Examples:
    SCHTASKS
    SCHTASKS /?
    SCHTASKS /Run /?
    SCHTASKS /End /?
    SCHTASKS /Create /?
    SCHTASKS /Delete /?
    SCHTASKS /Query  /?