Customizing existing forms in Drupal

The Form API does not just provide a way to create forms. There are ways to alter existing forms through hooks in a custom module. By using this technique, new elements can be added, default values can be changed, and elements can even be hidden from view to simplify the user experience.

The altering of a form does not happen in a custom class; this is a hook defined in the module file. In this recipe, we will use the hook_form_FORM_ID_alter() hook to add a telephone field to the site’s configuration form.

How to do it…

1. Ensure that your module has a .module file to contain hooks, such as mymodule.module.
2. We will implement hook_form_FORM_ID_alter for the system_site_information_settings form:

   <?php

   use Drupal\Core\Form\FormStateInterface;

   function mymodule_form_system_site_information

     _settings_alter(array &$form, FormStateInterface

       $form_state) {

     // Code to alter form...