Fichier web/modules/custom/offer/src/Form/CustomConfigForm.php

namespace Drupal\offer\Form;
 
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
 
/**
 * Class CustomConfigForm.
 */
class CustomConfigForm extends ConfigFormBase {
 
    /**
     * {@inheritdoc}
     */
    protected function getEditableConfigNames() {
        return [
            'offer.customconfig',
        ];
    }
 
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'custom_config_form';
    }
 
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $config = $this->config('offer.customconfig');
 
        $form['analytics'] = [
            '#type'  => 'details',
            '#title' => $this->t('Marketing & analytics'),
            '#open'  => TRUE,
        ];
 
        $form['analytics']['tagmanager'] = [
            '#type'          => 'textarea',
            '#title'         => $this->t('Tagmanager code'),
            '#default_value' => $config->get('tagmanager'),
            '#maxlength'     => NULL,
        ];
 
        return parent::buildForm($form, $form_state);
    }
 
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        parent::submitForm($form, $form_state);
 
        $this->config('offer.customconfig')
            ->set('tagmanager', $form_state->getValue('tagmanager'))
            ->save();
    }
 
}

Fichier web/modules/custom/offer/offer.links.menu.yml

offer.config:
  title: 'Offer global settings'
  route_name: offer.config
  description: 'Global settings for the offer platform'
  parent: system.admin_config_system
  weight: 99

Fichier web/modules/custom/offer/offer.routing.yml

offer.config:
  path: '/admin/config/offer/adminsettings'
  defaults:
    _form: '\Drupal\offer\Form\CustomConfigForm'
    _title: 'Offer platform global settings'
  requirements:
    _permission: 'administer site configuration'

Lire une valeur de la configuration et la rendre disponible dans le template twig :

/**
* Implements hook_preprocess_html().
*/
function MONMODULE_preprocess_html(&$variables) {
    $variables['tagmanager'] = \Drupal::config('offer.customconfig')->get('tagmanager');
}

La variable tagmanager est intégrée au template.

{{ tagmanager }}