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I  call ajax from select2 in backoffice so I create hook

public function hookActionAdminControllerSetMedia() {
MediaCore::addJsDefL('my_search_token', $this->context->link->getAdminLink('Psb2BAjaxModuleAdminController')); $this->context->controller->addCSS(_PS_BO_ALL_THEMES_DIR_ . 'default/js/select2-full/dist/css/select2.min.css','all'); $this->context->controller->addJS(_PS_BO_ALL_THEMES_DIR_ . 'default/js/select2-full/dist/js/select2.min.js'); $this->context->controller->addJS(_PS_BO_ALL_THEMES_DIR_ . 'default/js/tree.js'); 
$this->context->controller->addCSS($this->_path . 'views/css/' . $this->name . '.css', 'all');
}

And In my Module psb2b/controllers/admin/ajax.php

namespace Company\PSB2B\Controller\admin;
use Symfony\Component\HttpFoundation\JsonResponse;

class Psb2BAjaxModuleAdminController extends ModuleAdminController 
{ 

public function initContent() { 
$this->ajax = true; 
parent::initContent(); 
return new JsonResponse('test'); 
}
}

And my js

$(document).ready(function(){ 
  $('#category_features').select2({ 
    width: 'resolve', 
    ajax: { 
      type: 'POST', 
      url: my_search_token, 
      dataType:'json', 
      delay: 250, 
      data: function (params) { 
        return { 
          q: params.term // search term 
               }; 
      }, 
      success: function (result) 
      { console.log(result); 
      } } 
  }); 
});

But I got error message Controller Psb2BAjaxModuleAdminController not found