how to add an event subscriber when the user reaches the login form

I am new on Drupal 8 , I have implemented a hook ‘hook_form_alter’ in my module to add a new field to the login form , but now I want to handle it with the event , I want to trigger an event when the user reaches the login form and then add this field using event not a hook.

Can anyone please suggest me how to do it ?

This is the hook

function my_module_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {
  if ($form_id == 'user_login_form' ) {
    // Add placeholders.
    $form['condition'] = array(
      '#type' => 'checkbox',
      '#options' => t('Enable'),
      '#title' => t('J’accepte les conditions'),
      '#required' => true,
    );
  }
}