MigrationConfigurationForm.php

<?php

namespace Drupal\foo_migrate\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\migrate_plus\Entity\Migration;

/**
 * Interactive configuration of the blah migration process.
 */
class MigrationConfigurationForm extends FormBase {

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $video_full_migration = Migration::load('video_full');
    $source = $video_full_migration->get('source');
    if (!empty($source['urls'])) {
      if (is_array($source['urls'])) {
        $default_value = reset($source['urls']);
      }
      else {
        $default_value = $source['urls'];
      }
    }
    else {
      $default_value = 'http://example.com/f/foo/bar';
    }
    $form['url'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Service endpoint for retrieving video data'),
      '#default_value' => $default_value,
    ];

    $form['actions'] = ['#type' => 'actions'];
    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
      '#button_type' => 'primary',
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $video_full_migration = Migration::load('video_full');
    $source = $video_full_migration->get('source');
    $source['urls'] = $form_state->getValue('url');
    $video_full_migration->set('source', $source);
    $video_full_migration->save();
    drupal_set_message($this->t('Import configuration saved.'));
  }

}