How to do dependency injection on complex classes like when extending WidgetBase
You will need to implement ContainerFactoryPluginInterface in order to use dependency injection in your class.

Include use Drupal\Core\Plugin\ContainerFactoryPluginInterface; and implement the interface so it looks like:

class XoisSicSelect extends WidgetBase implements ContainerFactoryPluginInterface {

    ....

}


You will also need to make sure your __construct() and create() methods contain the correct parameters. 

If you look at ContainerFactoryPluginInterface.php, you will see the parameters the create() method takes in:

public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition);


Thus, your final code should look like:
    

use Drupal\Core\Field\FieldDefinitionInterface;
use Drupal\Core\Field\WidgetBase;
use Drupal\xois\SicsService;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;

class XoisSicSelect extends WidgetBase implements ContainerFactoryPluginInterface {
  /**
   * SICs service.
   *
   * @var \Drupal\xois\SicsService
   */
  protected $sicsService;

  /**
   * {@inheritdoc}
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, SicsService $sicsService) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
    $this-&gt;sicsService = $sicsService;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $plugin_id,
      $plugin_definition,
      $configuration['field_definition'],
      $configuration['settings'],
      $configuration['third_party_settings'],
      $container-&gt;get('xois.sics')
    );
  }
}