using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var body = "Hello, world welcome. [TKNM-1234] Blah blah [HelpCode-5] Blah blah";

        var ticketnumbers = body.Split().Where(x => x.StartsWith("[TKNM") && x.EndsWith("]")).Select(x=>x.Replace("[", string.Empty).Replace("]", string.Empty).Split('-')[1]).ToList();

        Console.WriteLine(string.Join(", ", ticketnumbers));

    }

}