using System;

using System.Linq;

​

public class Program

{

    public static void Main(string[] args)

    {

        const string value = "Hi!";

​

        var chars = value

            .Select(c => (int) c)

            .Select(c => $@"\u{c:x4}");

​

        var result = string.Concat(chars);

​

        Console.WriteLine(result);

    }

}