Short answer: you don't.
Long answer: from the man pages:
apt (Advanced Package Tool) is the command-line tool for handling
packages. It provides a commandline interface for the package
management of the system. See also apt-get(8) and apt-cache(8) for
more low-level command options.
apt-get is the command-line tool for handling packages, and may be
considered the user's "back-end" to other tools using the APT
library. Several "front-end" interfaces exist, such as aptitude(8),
synaptic(8) and wajig(1).
apt-get are two tools which are part of the
apt can be used to perform some functions and
apt-get others. They have some overlap.
apt covers all the common options a basic user would need.
apt-get contains more low-level functions that a sysadmin would use.
As per the full man page of the
apt tool, it does not have a command to auto remove unused package. One has to use
apt-get (or possibly another front-end interface).