Add separator

Given a string and a separator, write a method that adds separator between each adjacent characters in a string.
Expected input and output
AddSeparator("ABCD", "^") → "A^B^C^D^" AddSeparator("chocolate", "-") → "c-h-o-c-o-l-a-t-e"
using System;

namespace CSharpExercises.Exercises.Strings
{
    class AddSeparatorTask
    {
        static string AddSeparator(string word, string separator)
        {
            string separatedWord = string.Empty;

            for (int i = 0; i < word.Length; i++)
            {
                separatedWord += i < word.Length - 1 ? word[i] + separator : word[i].ToString();
            }

            return separatedWord;
        }

        static void Main(string[] args)
        {
            Console.WriteLine(AddSeparator("computer", "_"));  // c_o_m_p_u_t_e_r
            Console.WriteLine(AddSeparator("$*(#", " "));       // $ * ( #
            Console.WriteLine(AddSeparator("AC", "B"));        // ABC
            Console.WriteLine(AddSeparator("octopus", "X"));   // oXcXtXoXpXuXs
        }
    }
}