Recently Combine method was added to Energy.Core package, so you might want to use it to join URL parts.
string url;
url = Energy.Base.Url.Combine("https://www.youtube.com", "watch?v=NHCgbs3TcYg");
Console.WriteLine(url);
url = Energy.Base.Url.Combine("https://www.youtube.com", "watch?v=NHCgbs3TcYg", "t=150");
Console.WriteLine(url);
Additionally it will recognize parameter part, so it will work as you might expect (joining path with slash and parameters with ampersand).