How to add a checkbox to form builder component settings form?
you have to do it inside the form_id 'form_builder_field_configure'.
Example : 

hook_form_alter($form,$form_state,$form_id){
`switch($form_id){
     case 'form_builder_field_configure':
     // You put your code here
     $form['check'] = array(
       '#form_builder' =&gt; array(
         'property_group' =&gt; 'validation'
       ),
       '#type' =&gt; 'checkbox',
       '#title' =&gt; t('Check'),
       '#default_value' =&gt; '',
       '#description' =&gt; t('Check this option if the user should be able 
    to upload 
     files.'),
      '#weight' =&gt; -1,
);
  break;
 }


}
     `

I hope it'll help you.