You can log directly into Plesk using your hosting account's username and password by going to:
https://[your hosting account's IP address]:8443
You can proceed past any security warning your Web browser displays.
If you don't know your hosting account's IP address, see this article.
For help with your Plesk hosting account username and password, see:
You can use this method of logging in to give someone access to your hosting account without giving them access to your customer account.