Steps to create sub domain in Plesk:

1. Login to plesk control panel

2. Click Home

3. Select Sub domain >> Add New Sub domain >> Select Physical Hosting

4. Enter the sub domain name,  for eg : xxx.domain name.com

5. If you need  separate ftp user name and password for the sub domain >> select Create a separate FTP user account for this subdomain

6. If you need same ftp user name and password as per your main domain >>  select Use the FTP user account of the main domain

7. Click ok

8. After that update A record for the sub domain to view your sudb domain in your browser.

Now sub domain is created successfully for your main domain.