designtopack/public/site/plugins/comments/routes/create.php

<?php

return [
  'pattern' => '(:all)create-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 = $file->comments()->isEmpty() == true ? [] : Yaml::decode($file->comments()->value());

      $newComment = [
        'page' => [
          'uri' => (string) $page->parent()->uri(),  
          'title' => (string) $page->parent()->title(),
        ],
        'file' => [
          'uuid' => (string) $file->uuid(),
          'pageIndex' => $data->filePageIndex,
        ],
        'position' => [
          'x' => null,
          'y' => null
        ],
        'replies' => [],
        'text' => $data->text,
        'user' => [
          'name' => (string) $user->name(),
          'email' => (string) $user->email(),
          'uuid' => (string) $user->uuid(),
          'role' => (string) $user->role()
        ],
        'date' => (string) $data->date,
        'id' => $data->id,
        'type' => 'comment'
      ];

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

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

      return getFileData($newFile);
  }
];