designtopack/public/site/config/routes/add-comment.php

<?php

return [
  'pattern' => '(:all)add-comment.json',
  'method' => 'POST',
  'action'  => function () {
      $json = file_get_contents('php://input');
      $data = json_decode($json);

      $page = page($data->pageUri);
      $file = $page->file($data->fileName);
      $user = kirby()->user($data->userUuid);


      $comments = $file->comments()->isEmpty() == true ? [] : [];

      $newComment = [
        'fileUuid' => (string )$file->uuid(),
        'page' => $data->targetPage,
        'text' => $data->text,
        'username' => $user->name()->isNotEmpty() ? (string) $user->name() : (string) $user->email(),
        'date' => (string) $data->date,
        'id' => $data->id,
      ];

      $comments[$data->targetPage][$data->id] = $newComment;
      
      $newFile = $file->update([
        'comments' => $comments
      ]);

      $user->sendNotification('comments', $newComment);

      return getFileData($newFile);
  }
];