$form = $this->formBuilder->getForm('DrupalxyzFormsXyzActionsForm',$parameter);

return [
  '#form'=>$form
]

$form = $this->formBuilder->getForm('DrupalxyzFormsXyzActionsForm',$parameter);

$form = render(form);

$form .= '<div>custom html </div>';

return [
  '#markup'=>$form
]