ResolutionPlesk uses for login into CP an URL as on example below:
Where 'DOMAIN.TLD' - Parallels Plesk Panel server hostname or IP, 'admin' - user name you wish to login with, 'setup' - the user's password and the 'en_US' is locale.
In the URL above replace password with correct one.
You can write your own login page and create the form that will post these data to Plesk login page (login_up.php3). Also, you can use Plesk login page (https://DOMAIN.TLD:8443/login.php3) HTML sources as example, open it in the browser and save the central frame that contains login and password input fields.