You can access your customer portal from below client area:-

1. Go to Client Area, then login with your email address and password.

2. Click on "Services" and "My Services", followed by the Plesk License key product

e.g Plesk Web Admin 10 Domains for Dedicated Server

3. Scroll down the webpage to the bottom until you see "License Details"
The Product Key will be shown. Enter this key to activate your Plesk Control Panel
e.g A00B00-xxxxxx-xxxxxxx-xxxxx-xxxxxx