public function buildForm(array $form, FormStateInterface $form_state){
    $form = array();

    $form['adherent_id'] = array(
        '#type' => 'email',
        '#prefix' => '<div  class="db f5 dark-gray text textinp textinp2">Adherent</div >',
        '#default_value' => '',
        '#required' => TRUE,
        '#autocomplete_route_name' => 'simulation.autocomplete',
        '#attributes' => array(
            'id' => 'AdherentId',
            'class' => array('b', 'pa3', 'input-reset', 'ba', 'bg-transparent', 'hover-bg-moon-gray', 'hover-black', 'w-100'),
        ),
    );

    return $form;
}

simulation.autocomplete:
   path: '/simulator/autocomplete'
   defaults:
      controller: 'DrupalsmenoControllerAdminController::autocomplete1'
   requirements:
      _access: 'TRUE'

<?php

namespace DrupalsmenoController;

use DrupalCoreControllerControllerBase;
use SymfonyComponentHttpFoundationRequest;
use SymfonyComponentHttpFoundationJsonResponse;

class AdminController extends ControllerBase {

   function autocomplete1(request $request){
       $typed_category = $request->query->get('q');
       $matches = [
           '0' => 'user@mail.com',
           '1' => 'nina@mail.com',
           '2' => 'pierre-jr@mail.com'
       ];
       return new JsonResponse($matches);
   }

}