How to send a form automatically?

Hello, I have a form and I have to check the username and password on my DB. If all is ok, then, I have to send by another form this usernma and password to a CGI on a gateway. But this second part I would want to be automatically, I mean, I want to create the structure of the form that the CGI is waiting for but I don't want to show it to the user. Only send it to the CGI automatically. I don't know the structure of the CGI, it's built-in on the gateway. How can I do this? Thanks, Vicente