Untitled

use Drupal\captcha\CaptchaPointInterface;

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  // Replace 'your_custom_form_id' with the actual form ID of your custom contact form.
  if ($form_id == 'your_custom_form_id') {
    // Set the CAPTCHA type for the form.
    $form['captcha'] = [
      '#type' => 'captcha',
      '#captcha_type' => 'image_captcha/Image',
    ];
  }
}