Create registered user via admin

Is it possible to setup member pages with passwords via admin login instead via standard registration form? I am getting a lot of signups form dubious emails (hackers?)

If you disable user registration on

Dashboard > System & Settings > Registration > Login & Registration > Account Options > Allow visitors to signup as site members? > Off - only admins can create accounts from Dashboard

then anyone visiting the you-site/login page will see a login form but without registration.

Can I send registration info to user upon OK of adding user? What file would I alter or is this done on-the-fly?

I know how to do that programmatically from a package controller:

public function addEventListeners()
    Events::addListener('on_user_add', function($event) {
        $pkg = $this->app->make(PackageService::class)->getByHandle($this->pkgHandle);
        $my_functions = $this->app->make(MyFunctions::class, ['pkg' => $pkg]);
        $ui = $event->getUserInfoObject();

but I don’t know where exactly you can stick that code to override the core functionality without a package. It has to be a controller which will listen to events.