Note \ is an escape in C # use "extranet\\Admin" and the CMS administrator is sitecore \ admin
I assume that you need to know your extranet administrator. this is a good idea for creating roles, there may be several administrators (not sure if the IsAdministrator property works well for an extranet)
Sitecore.Context.User.IsInRole("extranet\\your extranet admin rol");
If you do not have an Extranet administration command and you do not want it, you can use what you already have if (Sitecore.Context.User.Name == "extranet\\Admin")
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