Hyperlink in email

I am trying to include a lnk to a webpage in emails sent to users when they have successfully registered on my site. In order to make the link unique and so I can also expire it in a day, I am adding a paramreter to the link with a random number as an Id that will allow me to identify them and complete the registration. I cann insert the link ok, but I am having trouble trying to add a variable random number.  Any one know how to do this?
My attempt.

Random rnd = new Random(); int rng = rnd.Next(5000); string rnum = rng.ToString(); string hlink = "<a href='http://www.ultramidwest.com/Account/confirmrider.aspx?id='" + rnum + "'/>"; ml.SendMailHtml(email, "Please navigate to '" + hlink + "' and login with the credentials you saved on the site", "Welcome");