Drupal form widget to preload with current user's IP.

CurrentUserIpWidget.php

<?php

namespace Drupal\se_core\Plugin\Field\FieldWidget;

use Drupal;
use Drupal\Component\Utility\Color;
use Drupal\Component\Utility\NestedArray;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\se_core\Seao;

/**
 * Plugin implementation of the 'current_user_ip' widget.
 *
 * @FieldWidget(
 *   id = "current_user_ip",
 *   module = "se_core",
 *   label = @Translation("Current User IP"),
 *   field_types = {
 *     "string"
 *   }
 * )
 */
class CurrentUserIpWidget extends WidgetBase {

  const IP_REGEX = '(\b25[0-5]|\b2[0-4][0-9]|\b[01]?[0-9][0-9]?)(\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}';

  /**
   * {@inheritdoc}
   */
  public function formElement(
    FieldItemListInterface $items,
    $delta,
    array $element,
    array &$form,
    FormStateInterface $form_state
  ) {
    $default_value = Seao::surveyResponses()->getDefaultIP();
    $value = $items[$delta]->value ?? $default_value;
    $element += [
      '#type' => 'textfield',
      '#placeholder' => $default_value,
      '#default_value' => $value,
      '#element_validate' => [[$this, 'validate']],
    ];
    $readonly = $this->getSetting('readonly');
    if ($readonly) {
      $element['#attributes']['readonly'] = TRUE;
    }

    return ['value' => $element];
  }

  /**
   * Validate the color text field.
   */
  public static function validate($element, FormStateInterface $form_state) {
    $value = NestedArray::getValue($form_state->getValues(), $element['#parents']);
    if (strlen($value) === 0) {
      $form_state->setValueForElement($element, '');

      return;
    }
    if (!preg_match('#' . self::IP_REGEX . '#', $value)) {
      $form_state->setError($element, t('@value does not appear to be a valid IP.',
        ['@value' => $value]));
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
        'readonly' => FALSE,
      ] + parent::defaultSettings();
  }

  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['readonly'] = array(
      '#type' => 'radios',
      '#title' => t('Read only'),
      '#options' => [0 => t('False'), 1 => $this->t('True')],
      '#default_value' => (int) $this->getSetting('readonly'),
    );

    return $element;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $bool_value = $this->getSetting('readonly') ? $this->t('TRUE') : $this->t('FALSE');
    $summary[] = t('Readonly: @value', ['@value' => $bool_value]);

    return $summary;
  }

}