Admin custom field

Questions
1

Silverstripe Version:
4.2.1

Question:

I’d like to be able to add a custom field to a model admin in order to save a custom message which will then be displayed/sent to all members. So far, I’ve achieved the display of the field by extending SilverStripe\Admin\SecurityAdmin using:

    function getEditForm($id = null, $fields = null){
      $form = parent::getEditForm($id, $fields);
      $membersgrid = $form->Fields()->dataFieldByName("Members");
      $form->Fields()->insertBefore("Members",TextareaField::create('CustomMsg'));
      return $form;
    }

How do I save any data to that field? Simply adding

private static $db = [
  'CustomMsg' => 'Text'
];

doesn’t create a field, nor can I save changes. It should function in a similar way to extending SiteConfig. Any ideas?