Let’s learn how to create a custom form in Drupal 9. If you know already how to create a custom form in Drupal 8 there is not much different, it’s almost the same.

First, you need to create a custom module to create a form. To create a custom module in Drupal 9 is something similar to Drupal 8. 

1) Creating a module in Drupal 9

Go to your drupal path and create a folder called ‘my_custom_form’: /drupal9/module/custom/my_custom_form

then, create a file info.yml file

i) my_custom_form.info.yml

name: 'my_custom_form'
type: module
description: 'Custom form using a custom module'
core: 8.x
package: 'Custom'
core_version_requirement: '^8 || ^9'

So the only difference from Drupal 8 in the info.yml is ‘core_version_requirement‘. Without this line, the custom module will not work in Drupal 9.

2) Creating a form

The second step is to create a custom form, for that’ we need a routing file, so let’s create one

Go to the path: /drupal8/module/custom/my_custom_form and create a route file:

ii) my_custom_form.routing.yml

my_custom_form.my_form:
  path: '/myform'
  defaults:
    _form: '\Drupal\my_custom_form\Form\MyForm'
    _title: 'MyForm'
  requirements:
    _access: 'TRUE'

As you can see there is no difference from Drupal 8. Drupal 9 routing.yml is the same as Drupal 8. 

After creating the above file, create two folders src and Form inside my_custom_form, So the structure would look like this now,

Inside the ‘Form’ folder create a PHP Class and put the below code:

iii) MyForm.php

<?php

namespace Drupal\my_custom_form\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Class MyForm.
 */
class MyForm extends FormBase {


  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'my_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['name'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Name'),
      '#description' => $this->t('Enter your fullname'),
      '#maxlength' => 64,
      '#size' => 64,
      '#weight' => '0',
    ];
    $form['phone'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Phone'),
      '#description' => $this->t('Enter your phone number'),
      '#maxlength' => 64,
      '#size' => 64,
      '#weight' => '0',
    ];
    $form['email'] = [
      '#type' => 'email',
      '#title' => $this->t('Email'),
      '#description' => $this->t('Enter your email'),
      '#weight' => '0',
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Display result.
    foreach ($form_state->getValues() as $key => $value) {
      \Drupal::messenger()->addMessage($key . ': ' . $value);
    }

  }

}

Just go through the above code, to create a form, you need to extend “FormBase” which is the core class and the form interface.

Again the only difference in the above code is instead of drupal_set_message() (which is deprecated in Drupal 9), I have used \Drupal::messenger()->addMessage().

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

When you use FormStateInterface, you should use the following functions as well getFormId(), buildForm(), and submitForm() without these functions a form cannot be created.

getFormId() – this is where you have to mention your form id and it should be unique
buildForm() – this is where you will create your form
validateForm() – here you can write your form validations
submitForm() – this is where you can handle submit processes

Now you have your custom module and a custom Form created, go to “Extends” and enable the module and then access the route path.

https://localhost/drupal/myform to see your custom form

You can download this Drupal 9 custom module with a form in my GitHub page,