You're out of luck unless they allow to use a full cgi-bin (many free servers limit what their cgi-bin can actually do). You need to be able to run a server-side script (perl, php, whatever...) that has permissions to write a flat file on the server.
There are free hosting companies that give you full cgi-bins. I think if you look back through ther posts in here you'll find urls to some of them.