how to add an event subscriber when the user reaches the login form
You can't, not with core alone. D8 doesn't have a form-alter event.
D8 added events as an alternative to some D7 hooks but didn't fully replace all the hooks available. Replacing all hooks was mooted for a D10 goal but didn't make the cut.
As such, the only way to do this modify core functionality like:

Patch core (see issue comment) to add form-alter events.
Do something like Hook Event Dispatcher which wraps custom form-alter events around a hook_form_alter handler.

Then add an event subscriber for it.