Hallo David,
this information is 8-10 months old, so it could have changed in the meantime...
But when we started with AEM Assets and wanted to setup the native integration, I had several sessions with an Adobe professional.
Since the AEM Assets application in Fusion didn't offer a lot of functionality and apparently still doesn't, we used the AEM Custom API call module in combination with the Workfront Custom API module.

The response is processed in a Create JSON module using the following Data structure:

The most tricky part is the setup of the JSON module:

Especially the fields documentProviderID and the externalStorageID.
Unfortunately I do not find, where I found the documentProviderID, but I remember, that this can be found using the api-unsupported.
I will add a solution, if I find it.
Finally the custom API call on Workfront looks like this:

Perhaps this helps.
Regards
Lars