How to add a placeholder field to Entity Forms
You should use  Field placeholder
module. 


  Field Placeholder module gives support to add HTML5 placeholder
  attribute to field widget forms.
  
  By default, this module integrates text, number and mail modules, but
  any other widget type can be added through
  hook_field_placeholder_info().
  
  Field placeholder adds a new option in field settings page to enter
  the placeholder value, which is included automatically when the widget
  is rendered.