drupal 7 add element after the username input text element on login form
You need to add suffix element to username field in login form. You can do it using hook_form_alter(&amp;$form, &amp;$form_state, $form_id)

Implement above hook in your custom module

function YOURMODULENAME_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'user_login_block' || $form_id == 'user_login') {
    $form['name']['#suffix'] = '@mycompany.com';
  }
}