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It's actually pretty easy. As Relate+ sample site does not create page types from code, all you need to do is to add property directly to the page via UI:
Then you need to add property to template markup anywhere you want to content area to show up:
<EPiServer:Property ID="epiSampleArea" PropertyName="SampleArea" runat="server" DisplayMissingMessage="false" />
Then you can see content area in edit mode:
Actually what I would do is to use excellent plugin made by Alf (http://talk.alfnilsson.se/2013/03/08/creating-page-type-classes-from-page-types-in-episerver-cms-7/) to reverse engineering page types back to code and add ordinary property to the page type class.
I’m playing around the Relate+ demo site and would like to know how to add ContentArea to the start page of the demo site (my intention was to add blocks to this ContentArea)