Cannot save Project

When adding a layer from our Portal (that is configured with Essentials for administration) the saving Project dialogue just hangs.

 

The viewer error logs states:

 

Error: ProjectConverter.fromGcxMapService: Services added through layer addition dialog are only supported if they have been whitelisted by your administrator.

 

Where/how do we whitelist layers? The root domain for our Portal is already listed under 'Trusted URLs' in Essentials Manager.

 

Would we need to add the entire service URL as a 'Trusted URL'?