How to add a field to menus?
I figured out how to add a field to the menus (not menu links, the actual menus). I use the machine name of the menu's title as a key and store the value with the key I use in the $form variable.
Here is my code:
function myModule_form_menu_edit_form_alter(&amp;$form, $form_state, $form_id) {
  $config = \Drupal::service('config.factory')-&gt;getEditable('myModule.settings');
  $my_field = $config-&gt;get(str_replace('-', '_', $form_state-&gt;getformObject()-&gt;getEntity()-&gt;id()));

  $form['my_field'] = array(
  '#type' =&gt; 'textfield',
  '#title' =&gt; t('Title'),
  '#default_value' =&gt; $my_field,
  '#weight' =&gt; '0',
  '#required' =&gt; FALSE,
  );

  $form['actions']['submit']['#submit'][] = 'myModule_custom_submit';
}

function myModule_custom_submit($form, $form_state) {
  $key = str_replace('-', '_', $form_state-&gt;getformObject()-&gt;getEntity()-&gt;id());
  $config = \Drupal::service('config.factory')-&gt;getEditable('myModule.settings');
  $config-&gt;set($key, $form_state-&gt;getValue('my_field'))-&gt;save();
}