using System;

public class Program

{

    public static void Main()

    {

        var originalUri = new Uri("http://www.contoso.com:8080/");

        var url = "~/blablabla/toto.png";

        var absoluteUrl = url.Replace("~/", $@"{originalUri.Scheme}://{originalUri.Authority}/");

        Console.WriteLine(absoluteUrl);

    }

}