To get the user to enter that information, you can use our "variations" feature
http://www.e-junkie.com/ej/help.php) which will let you create a text field with the purchase button.
As for backend, we don't host the code generators. You will need to host the Java file on your website and use our integration feature (
http://www.e-junkie.com/ej/help.integration.php) to post data to it. It will be easier for you if you'd rather get a PHP keygen from iphonesimfree.