Adding post with thumbnail image to WordPress blog with JoeBlogs / C#

I’m trying to add new post with thumb image but i still couldnt get success on it.

 public static int addContent(int postType, string title, string body, string post_thumbnail, string[] categories, string[] tags, DateTime dateCreated)
{
  Post post = setPost(postType, title, body, post_thumbnail, categories, tags, dateCreated);
  using (var wrapper = getWordPressWrapper())
  {
    return wrapper.NewPost(post, true);
  }
}

get setPost method,

public static Post setPost(int postType, string title, string body, string post_thumbnail, string[] categories, string[] tags, DateTime dateCreated)
{
  string type = postType == 1 ? "post" : "page";
  var customFields = new CustomField[] { 
    new CustomField() { 
        // Don't pass in ID. It's auto assigned for new custom fields.
        // ID = "name", 
        Key = "post_thumbnail", 
        Value = post_thumbnail
    } 
    //,
    //     new CustomField() { 
    //    // Don't pass in ID. It's auto assigned for new custom fields.
    //    // ID = "name", 
    //    Key = "post-thumbnail", 
    //    Value = post_thumbnail
    //}  ,
    //     new CustomField() { 
    //    // Don't pass in ID. It's auto assigned for new custom fields.
    //    // ID = "name", 
    //    Key = "post-thumbnails", 
    //    Value = post_thumbnail
    //} ,
    //     new CustomField() { 
    //    // Don't pass in ID. It's auto assigned for new custom fields.
    //    // ID = "name", 
    //    Key = "thumbnail", 
    //    Value = post_thumbnail
    //} 
  };
  return new Post
  {
    PostType = type,
    Title = title,
    Body = body,
    Categories = categories,
    Tags = tags,
    DateCreated = dateCreated,
    CustomFields = customFields
  };
}

as you see its commented. I tried everything to post thumb images.

I’ve tried post_thumbnail, post-thumbnail, post-thumbnails, thumbnail keys but nothing happen.

How can i do that?