code: public function onPageRendering(&$templateName, array &$twigVariables)
      {
          $twigVariables['auth']['plugin'] = $this;
          $twigVariables['auth']['vars'] = $this->output;
  
          // Variables useful only in successful execution
          if (!$this->errorOccurred) {
              $twigVariables['auth']['user'] = $this->user;
  
              // Previous form submission
              $old = $this->session->getFlash('old');
              if (count($old) && isset($old[0])) {
                  $twigVariables['auth']['old'] = $old[0];
              }
          }
      }
docs: Pico API event
  
  Makes certain variables accessible in the twig templates
  
  @param string $templateName Name of the twig template file
  @param array $twigVariables Twig variables