1. Login to Plesk.
2. If you are in the admin area, click on ‘Home’. Next, click on the domain name for which you want to set the directory index.
3. Click on the ‘Websites & Domains’ tab.
4. Click on the ‘Show Advanced Operations’.
5. Click Virtual Directories.
6. Click Manage for the affected domain.
7. Click Directory Properties
8. Add the file name if it is not there, and then rearrange the order if needed.
9. Click OK to apply the changes.