Need a prepaid code plugin

I’m looking for a plugin/widget that allows a user to enter a certain code in a field and takes them to a url. I run a membership website and users pay online. But often times I meet customers face to face and they pay right then and there. So I give them a “prepaid code” to enter on the website and it redirects to a members-only page.

Is there a plugin that will allow me to do this? I tried using contact form 7 and it works fine with the whole redirect but I can’t get it to accept ONLY A CERTAIN value in the field which is the prepaid code.