Before you add any domains or websites for a customer, that customer account must be added in Plesk in order to manage the websites associated with the customer through Plesk easily.
Login to the Plesk with admin user.
Once logged in, on the left hand side under Hosting services section click on Customers.
Now on the next screen click on Add New Customer button.
On the next step, enter the contact information of the customer for which the hosting account is being set up for.
Scroll down a bit. Under “Access to Panel” section create a username and password for the customer.
Again scroll a bit. Under Create subscription for the customer section provide all the details along with the website domain name, the hosting / service plan, , addons etc…
Once you provide all the details, click on the OK button at the bottom to add the hosting/customer account.
On the next screen you will see that your first customer / website is added to Plesk successfully.