1. Login to Plesk Admin https://IPAddress:8443 or https://hostname:8443
2. Next go to Websites & Domains for your Websites
3. Click on FTP Access as per below screenshot.
4. After that click on "Add an FTP Account"
5. Fill in the FTP Account detail example as per below.
6. Click OK once you have done.
7. Lastly it will appear to FTP account list of your domain as per below.
If you want to access FTP account through FileZilla may refer to this URL.