Login to Plesk control panel
Under Hosting Services click on Domains for which you want to create the password protected directory.
On the right hand you will see the list of the domains that are hosted on the server. Click on the domain for which you want to enable/disable ASP.NET support.
Now click Hosting Settings option for that domain.
On the Hosting Settings page scroll down a bit and under Web scripting and statistics option you will see the option for Microsoft ASP.NET support along with its version. If you remove the check sign the ASP support will be disabled & if marked the ASP support will be enabled.
After making your desire changes scroll down a bit and click on OK button to save the changes.