public partial class _Default : Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string destinationURL = "http://www.contoso.com/default.aspx?user=test";

        NextPage.NavigateUrl = "~/Finish?url=" + Server.UrlEncode(destinationURL);
    }             
}

public class SampleClass
{
    public string GetUrl()
    {
        string destinationURL = "http://www.contoso.com/default.aspx?user=test";

        return "~/Finish?url=" + HttpContext.Current.Server.UrlEncode(destinationURL);
    }
}

String TestString = "This is a <Test String>.";
StringWriter writer = new StringWriter();
Server.UrlEncode(TestString, writer);
String EncodedString = writer.ToString();