Follow the steps below to add custom error pages for your Windows hosting plan.
Step 1: Login to the Plesk panel of the domain name for which you wish to set custom error pages
Step 2: Add the custom error page
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Click on Virtual Directories.
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Click on Error Documents.
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Click on the error for which you wish to customize.
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Select URL in the Type menu and enter the path to your document in the Location field. The path should consist of "https://
" plus the path to the file. 
Example: Your domain name is example.com, and you have created a file named badrequest_400_1.html and saved it in the httpdocs\my_errors directory. To use this file as an error document, you need to type the following URL into the Location field: https://example.com/my_errors/badrequest_400_1.html.