Plesk and DNN are not good friends. If you have access to your server through RDP, I would suggest not using plesk at all...

Normally, you would just add the subdomains to your dns and point them to the same ip address. Then just add bindings in IIS. Plesk messes this up. The only way it would work is to create a so called domain alias, for the already existing domain. You cannot create a new account an point it to an existing domain...

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Erik van Ballegoij, Former DNN Corp. Employee and DNN Expert

DNN Blog | Twitter: @erikvb | LinkedIn:

I found it, It is very easy....

When you want add Parental Sub-Portal after adding on DNN, We have to goes to "Website & Domains" Tab of our account and then Click on "Add New Domain Alias" as you told before. But Remember there is a Check box that by default is checked. it is for Redirect to main Parent Domain as HTML 301 Code. we have to un-check that check box:
If uncheck, it will working, if check it will redirect parent domain.

[ * ] Redirect with the HTTP 301 code
When visitors open the alias's URL, they will be redirected to the URL of the primary domain. Plesk uses the search engine-friendly HTTP 301 redirection, which means the search engine ranking will not be split between the URLs.