Is there a method (within affiliates plugin or other wise)

Is there a method (within affiliates plugin or other wise) to force a site visitor to enter / use an affiliates referral code when signing-up on my signup (membership) page? Many of our affiliates do not have web sites and will simply refer people over. In that case, they might just give their friends their referral code on a card or something. a link would not be used.

I couldn't find what I need in affiliate, so I set up a page to have the visitor enter a referral code. I used the code entered and build a URL to include the REFERRALCODE, using to open my signup page with the REFERRALCODE at the end of the URL. I used onclick="self.location='' to build and open the signup page. The problem is using self.location, everything after the ? is dropped and so the REFERRALCODE doesn't get recognized when the page is brought up.

1. I have a Membership site that provides content for paid members.
2. I also use the Affiliate plugin so that anyone can market my site and be compensated for it.
3. Visitors can always view the general pages of the site, to gain a feel for what we offer.
4. Visitors will generally find us in one of three ways:
a) Referred by someone they know personally (could be an affiliate or not)
b) Find us through search engines,
c) Click on an affiliate link somewhere.