autofill user/pass at a non WP form with WP info

Descripción: I have a site [login to view URL]; and another site meant for my subscription users [login to view URL] At this subsite there is a login screen: user name and password User name is the wordpress username and the password is always a fixed, no matter the user. Freelance has to provide the code to use in wordpress, so user clicks on this code and in the background, user name and password is being filled, so user see only the resulting page ([login to view URL] = [login to view URL])