No query string was present - GraphQL & C# - PostRequest

Hi,
I have been struggling with fetching the data from monday.com using GraphQL, C#, PostRequest.
I have been getting 'No query string was present ’ but the account number is fetched correctly.
I’m new to Monday.com and currently working on new Feature to integrate Monday with our product.

the returned json object.
{“errors”:[{“message”:“No query string was present”}],“account_id”:XXX}

I hope someone can help with this Issue or guide me.
I’m using https NOT http

Thanks

public async Task GetBoardItems([FromQuery] long boardId)
{
string query = @“query {
    boards(ids: boardID){
    items {
        id
        name
        created_at
        creator_id
        column_values {
            id
            title
            value
            text
            }
        updates{
            item_id
            text_body
            created_at
            updated_at
        }
      }
    }
}”;

        query = query.Replace("boardID", boardId.ToString());

        var json = new JavaScriptSerializer().Serialize(query);
        var content = new StringContent(json, Encoding.UTF8, "application/json");
        var responseFromServer = await client.PostAsync(url, content);
        var contents = await responseFromServer.Content.ReadAsStringAsync();
        var boardItems = JsonConvert.DeserializeObject<Root>(contents);
        return boardItems;
    }