Build a From from another Form

Example : (From user login block)

  public function build() {

    $form = \Drupal::formBuilder()->getForm('Drupal\user\Form\UserLoginForm');

    //You can use Form Class (\Drupal\Core\Form\FormBuilderInterface) or The form ID, Like

    //$form = \Drupal::formBuilder()->getForm('user_login_form');

    unset($form['name']['#attributes']['autofocus']);

    // When unsetting field descriptions, also unset aria-describedby attributes

    // to avoid introducing an accessibility bug.

    // @todo Do this automatically in https://www.drupal.org/node/2547063.

    unset($form['name']['#description']);

    unset($form['name']['#attributes']['aria-describedby']);

    unset($form['pass']['#description']);

    unset($form['pass']['#attributes']['aria-describedby']);

    $form['name']['#size'] = 15;

    $form['pass']['#size'] = 15;

    $form['#action'] = $this->url('<current>', [], ['query' => $this->getDestinationArray(), 'external' => FALSE]);

    // Build action links.

    $items = array();

    if (\Drupal::config('user.settings')->get('register') != USER_REGISTER_ADMINISTRATORS_ONLY) {

      $items['create_account'] = \Drupal::l($this->t('Create new account'), new Url('user.register', array(), array(

            'attributes' => array(

              'title' => $this->t('Create a new user account.'),

              'class' => array('create-account-link'),

            ),

      )));

    }

    $items['request_password'] = \Drupal::l($this->t('Reset your password'), new Url('user.pass', array(), array(

          'attributes' => array(

            'title' => $this->t('Send password reset instructions via email.'),

            'class' => array('request-password-link'),

          ),

    )));

    return array(

      'user_login_form' => $form,

      'user_links' => array(

        '#theme' => 'item_list',

        '#items' => $items,

      ),

    );

  }