As far as I know custom auth is not supported in workflows at this time. Last I checked I couldn't find this stated in the docs either but when opening a ticket I saw
I executed the auth flow within the workflow for the time being. It's not ideal but works for now
@Etuzy As noted above, resources with custom authentication are not currently supported. This is something that we have on the roadmap and will be tackling. I don't have an ETA right now, but will update you all here as I get any additional information. I'll also move this over to Feature Requests. Thanks!