Finisher für Ext Form

public function executeInternal()
{
    $newField = $this->finisherContext
        ->getFormRuntime()
        ->getFormDefinition()
        ->getPageByIndex(0)
        ->createElement('firstname', 'Text')
    ;
​
    $this->finisherContext
        ->getFormRuntime()
        ->getFormState()
        ->setFormValue('firstname', 'Kurt');
}