There's no UI setting to control what the property is. Edit your project file in Notepad (or unload the project in VS and then right click and choose "edit"), and add this line:
<VSSDKTargetPlatformRegRootSuffix>Roslyn</VSSDKTargetPlatformRegRootSuffix>
into the appropriate PropertyGroup. Look in the projects created via any of the Roslyn templates for an example.