When purchasing a SSL cert, you will be required to submit a CSR or certificate signing request.
To generate a CSR from your server, kindly login to Plesk control panel and follow the steps below.
1. Click on "Tools & Settings".
2. Click on "SSL Certificates".
3. Click on "Add SSL Certificate" and fill-in the details.
4. Then click on "Request".
Click on the certificate name that you have just created and scroll down below, there you will see the CSR.