Step 1 : Login to the Plesk panel with the user credentials.
Step 2 : Go to Websites & Domains, choose the respective Domain.
Step 3 : From the respective domain page, select Password-Protected Directories and click the Add Protected Directory Option.
Step 4 : Specify the Directory in the Document Root which is to be secured and click OK.
Step 5 : Now, Select the directory name which was created.
Step 6 : Select Add a User option, and give the Username and Password for accessing the directory.
Step 7 : Verify the Protection by opening the URL in a browser and login using the credentials.