title: how to add checkbox in field settings form in Drupal 8
body: <p><a href="https://i.stack.imgur.com/wZPhS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wZPhS.png" alt="enter image description here"></a></p>
  
  <p>How Can I add custom check box under the default value fieldset and below the text field and it's custom submit handler?</p>
  
  <p>I have used hook_form_field_ui_field_edit_form_alter() but it's not works.
  I have used hook_field_widget_third_party_settings_form() but it's not working.</p>
  
  <p>Don't know what I am doing wrong. 
  Please help.</p>