key.php

<?php 

...
public function form()
{
  return $this
    ->scaffoldForm()
    ->update('fieldname', function ($e) {
      return $e->setInput((new \Terranet\Administrator\Form\Type\Key('fieldname')));
    });
}

readonly.php

<?php 

...
public function form()
{
  return $this
    ->scaffoldForm()
    ->update('fieldname', function ($e) {
      return $e->getInput()->setAttributes(['readonly' => true]);
    });
}

value.php

<?php 

use Terranet\Administrator\Form\Type\Hidden;

...
public function form()
{
  return $this
    ->scaffoldForm()
    ->update('fieldname', function ($e) {
      return $e->setInput((new \Terranet\Administrator\Form\Type\Hidden('fieldname'))->setValue(112));
    });
}