How can i get form / ActiveForm in controller

i have modal ajax

and want to send $form into controller

so i can use $form to render my modal

controller

public actionShowModal()

{

   ...

   return $this->renderAjax('_modal', ['form' => $form]);   <-- how i can get $form

}

view _modal.php

<?= $form-field($modal, 'name')->textField() ?>

public actionShowModal()

{

   // See your form here:

   Yii::$app->request->post();

}