Changing a Preprocessor DLL from Wix3 to Wix5. How to access in the Product.WXS?

[wjpoconnor]

My company is in the midst of upgrading our installations from Wix3 to Wix5. Part of that effort is to rewrite a DLL we created for handling some complicated file versioning. I believe that I have finished rewriting the DLL to use the WixToolSet.Extensibility package. I have a public class that implements the IExtensionFactory interface. It has the TryCreateExtension method that places a reference to the BasePreprocessorExtension class that implements our versioning code.
My question primarily concerns how to actually make use of the DLL in Wix5.
With Wix3, that version of the DLL was accessed like this:

<?define Increment = $(versionUtil.GetIncrement($(var.FirstProject.TargetPath))) ?>
<?define SvnRev = $(versionUtil.GetRevision($(var.FirstProject.TargetPath))) ?>

I can't find any documentation on how Wix5 equivalents are supposed to be included in the Wix build.

There also was a AssemblyDefaultWixExtension reference in the Wix3 Assembly.cs, however that doesn't seem to be supported anymore.

//add Wix Default Extension
[assembly: AssemblyDefaultWixExtension(typeof(VersionWixExtension.u_VersionWixExtension))]

Is there an equivalent in Wix5?

Attempting to build with the old references, causes a WIX0235 error during compilation:
WIX0235 Undefined preprocessor function '$(versionUtil.GetIncrement($(var.Console.TargetPath)))'.