How to hook the webmail login process ?

I would like to show a WebMail login form as part of a SSO /Oauth procedure in order to use cPanel Webmails for logging to external web services. Ideally the user would be redirected to an arbitrary url by the "client" application. On that url I would like to run some code that shows the standard cPanel webmail login and if the user logs in successfully the event is handled and the user is redirected to the "client" website instead of opening the webmail.