using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var data = "TOP:\nBOTTOM:";

​

        var rt = InjectAndPrint(data, "BOTTOM:", "<New Line>");

​

​

        rt.ForEach(x =>

        {

            Console.WriteLine(x);

        });

    }

    static List<string> InjectAndPrint(

        string textToSearch,

        string searchTerm,

        string textToInject)

    {

        var arr = textToSearch.Split('\n').ToList();

        arr.Insert(arr.IndexOf(searchTerm), textToInject);

        return arr;

    }

}