I would contact your host regarding this, as well as taking a look at some PLESK FTP Tutorials to get yourself familiar with the system. Have you tried asking your host for support?
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Many times but don't receive any valid help. Simply a link to a Microsoft site but the tutorial is about creating and managing Virtuals in the computer IIS and not in the Web Server IIS. I have the complete tutorial of PLESK but don't say nothing about this.