Alter field widget settings fields I'm trying to alter the settings fields displayed when you click on the cog icon on the &quot;Manage form display&quot; admin screen.
More specifically I want to alter fields set by another module through hook_field_widget_third_party_settings_form
I finally found the solution so i'll share it here as an answer.
It was not that hard, but I had to dig a bit into the core code in order to figure out how to do it.
The form_id we want to alter is entity_form_display_edit_form
Because the settings fields are loaded through ajax, we first have to check if we are currently editing a field settings by checking  $form_state-&gt;get('plugin_settings_edit')
The fields we are looking for are then located at  $form['fields'][FIELD_NAME]['plugin']['settings_edit_form']
function fintech_form_entity_form_display_edit_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  $field_name = $form_state-&gt;get('plugin_settings_edit');
  if(!empty($field_name) &amp;&amp; isset($form['fields'][$field_name]['plugin']['settings_edit_form'])) {
    $form['fields'][$field_name]['plugin']['settings_edit_form']['third_party_settings']['whatever_you_want_to_alter'] = '';
  }
}