// @nuget: HtmlAgilityPack

​

using System;

using HtmlAgilityPack;

public class Program

{

    public static void Main()

    {

        var html = @"<a class=""tlink"" href=""https://www.example.com/mypage.php?_src=ffk_title&amp;ffkid=66534&amp;site=data:http%3A%2F%2Fwww.othersite.com%2Frss%2F"" target=""_blank"">Click to get feed</a>";

​

        var htmlDoc = new HtmlDocument();

        htmlDoc.LoadHtml(html);

        var value = htmlDoc

            .DocumentNode

            .SelectSingleNode("//a")

            .Attributes["href"]

            .Value;

        Console.WriteLine(value);

    }

}