$fields['assigner'] = BaseFieldDefinition::create('entity_reference')
  ->setLabel(t('Assigner'))
  ->setDescription('The person who will issue this Task.')
  ->setRevisionable(TRUE)
  ->setSetting('target_type', 'user')
  ->setSetting('handler', 'default')
  ->setTranslatable(TRUE)
  ->setDisplayOptions('view', [
      'label' => 'hidden',
      'type' => 'author',
      'weight' => 0,
  ])
  ->setDisplayOptions('form', [
      'type' => 'entity_reference_autocomplete',
      'weight' => 5,
      'settings' => [
          'match_operator' => 'CONTAINS',
          'size' => '60',
          'autocomplete_type' => 'tags',
          'placeholder' => '',
      ],
  ])
  ->setDisplayConfigurable('form', TRUE)
  ->setDisplayConfigurable('view', TRUE);