Running a Drupal migration from form submit

migration_from_form_submit.php

    // You should probably just read Migrate Source UI code :) but ...

    $migration_id = $form_state->getValue('migrations');

    // Aha! We'll target the final migration, and let Migration
    // dependencies do the legwork of starting the other two.
    $migration_id = str_replace('node', 'media', $migration_id);

    /** @var \Drupal\migrate\Plugin\Migration $migration */
    $migration = $this->pluginManagerMigration->createInstance($migration_id);

    // Reset status.
    $status = $migration->getStatus();
    if ($status != MigrationInterface::STATUS_IDLE) {
      $migration->setStatus(MigrationInterface::STATUS_IDLE);
      drupal_set_message($this->t('Migration @id reset to Idle', ['@id' => $migration_id]), 'warning');
    }

    $options = [
      'file_path' => $form_state->getValue('file_path'),
    ];
    // Force updates or not.
    if ($form_state->getValue('update_existing_records')) {
      $options['update'] = TRUE;
    }

    $executable = new MigrateBatchExecutable($migration, new StubMigrationMessage(), $options);
    $executable->batchImport();