PWA List access via REST from sub site

I've got a Project server site that needs to access to the values of a list stored in the PWA when a new row is added in a list of its own.

For that I've created a workflow using Sharepoint Designer.

When I invoke http://...PWA/_api/web/Lists('GUID') from that workflow I always get the same error: Unauthorized. Even making the call using "App Step".

But if I invoke that service directly from chrome it works fine.

Any idea which permission is needed or what is happening? I've been looking the permissions and found nothing unusual.