<?php
namespace Drupal\drupalup_simple_form\Form;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\HtmlCommand;

/**
 * Our simple form class.
 */
class SimpleForm extends FormBase {
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'drupalup_simple_form';
  }


  /**
   * {@inheritdoc}
   */

 public function buildForm(array $form, FormStateInterface $form_state) {
  $form['search'] = array(
    '#type' => 'textfield',
    '#title' => t('Search:'),
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => $this->t('Submit'),
    '#button_type' => 'primary',
    '#ajax' => array(
      'callback' => [$this, 'submitForm'],
      'event' => 'click',
    ),
  );
  return $form;
}



  /**
   * {@inheritdoc}
   */

  //public function submitForm(array &$form, FormStateInterface $form_state) {
    // drupal_set_message($this->t('@can_name ,Your application is being submitted!', array('@can_name' => $form_state->getValue('candidate_name'))));
    // foreach ($form_state->getValues($results) as $result) {
      // print_r($json_string= (string) $results->getBody());
    // }
   // }

  /* public function submitForm(array &$form, FormStateInterface $form_state) {
    * drupal_set_message(
     *   $form_state->getValue($results)
      *  );
  * }
*/
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $url = 'http://systembevakningsagenten.se/api/json/1.0/searchProduct.json?query=';
    $term = $form_state->getValue('search');
    $query_url = $url. $term;
    $client =  \Drupal::httpClient();
    $request = $client->get($query_url);
    $data = json_decode($request->getBody(), TRUE);
    $results = [];
    if (!empty($data) && !empty($data['items'])) {
      foreach ($data['items'] as $item ) {
        $results[] = $item['name'];
      }
    }
     }

}
?>

Message: <input type="text" id="message">
<input type="submit" onclick='PostMessage()'>
<script>
function PostMessage() {
var comment = document.getElementById('message').value;
    window.location.assign('http://systembevakningsagenten.se/api/json/1.0/searchProduct.json?query='+comment)
}
</script>