Drupal 8: Add placeholder attribute to an input form element
Adding placeholder text to a Drupal 8 form element that is the label of the form can be done in your .theme file with the hook template_preprocess_input().
Documenting my findings about Drupal 8 here.
Adding placeholder text to a Drupal 8 form element that is the label of the form can be done in your .theme file with the hook template_preprocess_input().