From Parallels Tech Support:
In order to set the default page in Windows Plesk:
* Login to Plesk
* Click your domain
* Click Web Directories
* Click Directory Properties
In the option as Documents, you can set the order in which files are searched. If you want default.asp instead of index.html, you would make sure that the default.asp is listed above index.html. You can use the same option to set a non-existing file as the preferred filename. In this case if you check the 'Default Document' option for site in IIS you will see that default.asp enabled as the default document globally for the whole site and for /httpsdocs too.
Hope this helps!
Drew from Parallels
Twitter: @ParallelsPanel