How to attach a library from a custom FieldFormatter! :) Works

CustomFormatter.php

<?php
namespace Drupal\custom\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Field\FieldItemListInterface;

/**
 * Plugin implementation of the 'Custom' formatter.
 *
 * @FieldFormatter(
 *   id = "custom_formatter",
 *   label = @Translation("Custom Formatter"),
 *   field_types = {
 *     "string"
 *   }
 * )
 */
class CustomFormatter extends FormatterBase {
 public function viewElements(FieldItemListInterface $items, $langcode) {
    $element = [];

   foreach ($items as $delta => $item) {
      // Render each element as markup.
      $element[$delta] = [
        '#theme' => 'mycustom_theme',
        '#title' => 'hey hey',
        '#attached' => ['library' => ['custom/custom.mylibrary']]
      ];
    }
    return $element;
  }
}