<_CustomFiles> element not recognized in .pubxml

I'm trying to include a file in an ASP.NET filesystem publish that's not included in the solution (it's autogenerated from a post-build script). According to these instructions, I should be able to add a reference to the file in my .pubxml definition using the <_CustomFiles> element. Here's what my .pubxml file looks like:

<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <WebPublishMethod>FileSystem</WebPublishMethod>
    <LastUsedBuildConfiguration>Debug</LastUsedBuildConfiguration>
    <LastUsedPlatform>Any CPU</LastUsedPlatform>
    <SiteUrlToLaunchAfterPublish />
    <LaunchSiteAfterPublish>True</LaunchSiteAfterPublish>
    <ExcludeApp_Data>False</ExcludeApp_Data>
    <publishUrl>(publish URL here)</publishUrl>
    <DeleteExistingFiles>False</DeleteExistingFiles>
  </PropertyGroup>

  <PropertyGroup>
    <CopyAllFilesToSingleFolderForPackageDependsOn>
      CustomCollectFiles;
      $(CopyAllFilesToSingleFolderForPackageDependsOn);
    </CopyAllFilesToSingleFolderForPackageDependsOn>

    <CopyAllFilesToSingleFolderForMsdeployDependsOn>
      CustomCollectFiles;
      $(CopyAllFilesToSingleFolderForPackageDependsOn);
    </CopyAllFilesToSingleFolderForMsdeployDependsOn>
  </PropertyGroup>

  <Target Name="CustomCollectFiles">
    <ItemGroup>
      <_CustomFiles Include="scripts\app.min.js" />

      <FilesForPackagingFromProject  Include="%(_CustomFiles.Identity)">
        <DestinationRelativePath>Extra Files\%(RecursiveDir)%(Filename)%(Extension)</DestinationRelativePath>
      </FilesForPackagingFromProject>
    </ItemGroup>
  </Target>
</Project>

The second <PropertyGroup> and the <Target> elements were added by me.

However, Visual Studio isn't recognizing the <_CustomFiles> element and the file isn't being included in the publish. Notice the green squiggly line underneath the element:

When I hover over it, the following error message is shown:

The element 'ItemGroup' in namespace 'http://schemas.microsoft.com/developer/msbuild/2003' has invalid child element '_CustomFiles' in namespace 'http://schemas.microsoft.com/developer/msbuild/2003'

Why is this element not recognized by Visual Studio? What do I need to change in order to include this file in my filesystem publish?

Figured it out. I misunderstood the purpose of the <_CustomFiles> element. It seems this is a custom element (i.e. not part of the XML schema, hence the VS warning) used to pass information to the <FilesForPackaginFromProject> element below it.

It turns out this method was working for me, I was simply using the relative paths incorrectly.