code: public function clickSubmit($label = 'Submit', $additional = false)
      {
          if (! ($form = $this->page->getFormBySubmit(new SelectByLabel($label)))) {
              return false;
          }
          $success = $this->load(
                  $form->getAction(),
                  $form->submitButton(new SelectByLabel($label), $additional));
  
          return ($success ? $this->getContent() : $success);
      }
docs: Clicks the submit button by label. The owning form will be submitted by this.
  
  @param string $label    Button label. An unlabeled button can be triggered by 'Submit'.
  @param hash $additional Additional form data.
  
  @return string/boolean  Page on success.