As Mudassar said ... prevent somebody to type any URL is not possible ... But still you can achieve your goal via many techniques:
1- Via saving credentials in DB, then website confirms such credentials in order to open.
2- Via Request.QueryString, where SignUp process is hardcoded to produce some QueryString then Response.Redirect to the website. Website must confirm for proper QueryString before opening.
3- Via Session Variables, where SignUp process is hardcoded to produce some Session Variables then Response.Redirect to website. Website must confirm for existence of the Session Variables before opening.