I was trying the Forge ARKit API (https://app.swaggerhub.com/apis/cyrillef/forge-ar_kit/2.0.0#/info) and I was able to load a revit model in Unity through the asset (http://forgetoolkit.com/#/unityPackage). Then I saw that the API has an url to download a model as UnityPackage (https://app.swaggerhub.com/apis/cyrillef/forge-ar_kit/2.0.0#/ARKit/get_unity), but unfortunately I get a 501 status Not Implemented.
I'm doing a GET request to "https://developer-api-beta.autodesk.io/modelderivative/v2/arkit/{{urn}}/unity" with the Authorization token and the same urn base64url I used for the other urls.