It depends on when this command is supposed to run.
If it has to run when a particular url is requested, you should place in the action/controller where the command is supposed to run.
Why you don’t want to use active record? If you are approaching to Yii only yesterday, I advice you to use the most possible the framework instruments.
If you want to approach to a framework with your own programming style, you risk that the framework will be an obstacle instead of an help.
Better to do:
$user = new TblUser;
$user->username=$username;
$user->email=$email;
$user->save();