Check A Form And Trigger A Hidden Form To Become Visible ?

I want to get the code ( I cant write it Im still a newbie ) to check a form user/pass and trigger another hidden form.

Heres what going on:

Have created a form, Form 1 Login: it has a username and pass fields. It shares the page with a Members Login form.

I want javascript to check the user/pass on Form 1 Login against a user/pass I assign. I assume I can assign one in the Javascript code.

The script does not send the form to the server, it just checks to see that the user/pass agrees with what Ive assigned.

The script, if the user/pass is the same as I assigned, triggers a Registration form to pop up that Ive hidden with CSS left:-999em.

Now the script has fulfilled its function, its done. But heres what continues and Adobe BC does this part.

User fills out the pop up Registration form with any user/pass, their valid email and name.

Clicks Submit and the form sends the New user a new personal pass and new username via email.

User adds New user/pass to Members form and is allowed into the site.

This is a round about way but, I can not use php or server side in this process, BC does not allow server side only client side. So Im doing a work around. I know someone could find the Javascript even in a js folder. Tight security is not an issue. I researched this for weeks and BC tells me it will work with their system...I just dont know how to write it.