Auto Register and Form Submission with MySQL

I’ve built a form in Dreamweaver and I’m hosting it through Hostmonster which offers MySQL. I’ve linked my form with my host’s DB Table. I’ve built my membership site through WordPress.

My Question: When a user submits the form he or she will get a customized Thank You page. On that Thank You page (which isn’t in WordPress) I will include a link or button that will automatically bring them to the membership page without asking for registration. Somewhat like a universal secret password coming from my Thank You page.

I have a background form processing script that will email the user the randomly generated password, but I would like that username and password to be drop into a WordPress database and can be used for future logins into the membership section by the user.
I’m comfortable with creating a link to bring them to the membership page. But how do I connect my WordPress DB to my Host’s DB so when users log in the future they will have access with the password generated by my processing script not in WordPress.