function customusersubmit_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {

if ($form_id == 'user_login_form') {

//code to modify your form input

$form['name']['#attributes']['placeholder'] = t('username or emailaddress');
$form['name']['#title'] = 'Username or Email Address';

$form['actions']['submit']['#value'] = t('Publish');

$form['actions']['submit']['#submit'][] = 'custom_user_submit';
 //do something similar to create a custom validation handler
 }

function custom_user_submit(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {

 if($_GET['op']=='Publish') {

 drupal_set_message('working user', 'success');

  $cuser = $form_state->getValue('name');
  $cpass = $form_state->getValue('pass');

 }

 customusersubmit_user_login('$cuser', '$cpass');

 print_r(form_state);

 }