Wildcard Certificates use Subject Alternative Names (SANs) to secure a domain and all of its first-level subdomains. For example, a certificate for *.example.com secures www.example.com, mail.example.com, blog.example.com etc.
A standard SSL Certificate would only secure www.example.com, requiring you to purchase an additional certificate for mail.example.com and so on.

Source: https://www.digicert.com/wildcard-ssl-certificates.htm

1. Login to Plesk
2. Navigate to the Websites & Domains tab, select the domain you want to secure and click on Secure Your Sites link.
3. Click Add SSL Certificate button.
4. On the next page enter the Certificate Name to easily recognize the certificate among others on the server and fill in the required information for the CSR code.
5. Enter the FQDN for which the SSL will be activated (common name). The common name for Wildcard certificates should be represented with an asterisk in front of the domain ie. *.example.com
6. Click Request button.
7. A new record will be added to the list of SSL Certificates. Click on the newly created certificate to view the CSR code and RSA Private Key that were created.
8. You can now proceed in submitting the CSR to the SSL vendor.

1. Login to Plesk
2. Navigate to the Websites & Domains tab, select the domain you want to secure and click on Secure Your Sites link.
3. Select the SSL certificate that was created while generating the CSR code.
4. On the next page, paste ‘SSL certificate’ code in Certificate field and ‘intermediate CAs’ in CA certificate field.
   Note: depending on the file format customer has provided, you may need to convert the Intermediate CAs to PEM like PKCS#7 or P7B formats are not recognized by plesk. Use https://www.sslshopper.com/ssl-converter.html to convert such format.
5. After both fields are populated, click Send Text button.
6. Ensure import is successful and navigate back to Domain > Manage Hosting > Hosting Settings > tick SSL support check box > select Certificate Name dropdown and hit OK.
7. Verify SSL certificate was installed correctly in https://www.sslshopper.com/ssl-checker.html