You can achieve this by editing the Web Map JSON using an unsupported tool (ArcGIS Assistant) to directly update the copyright to include additional sources.
However I would NOT recommend this as editing Web Map JSON = unsupported and risky, it'll be difficult to maintain across maps or basemap changes, and as @abureaux mentioned you can enter some copyright grey areas.
The better way to do this is to have one or more of the following:
- Copyright at the item details page of all layers, maps and web apps
- Splashscreen in web app
- Layernames (legends)
- Text graphics or disclaimers as part of the web app interface
- Copyright disclaimers on print templates.
The layout view recommendation is for when creating layouts in ArcGIS Pro, i.e. to export maps to pdf. You can insert dynamic credit text to allow repositioning and resizing off the map itself. Though again you are technically required to identify the basemap sources in your pdf. The dynamic text just lets you decide how/where. This doesn't apply to web apps.
If you are creating a custom basemap layer to make your own custom basemap it is possible to specify the copyright in the service itself but I don't believe this is what you are doing and utilities are generally an operational layer, not a basemap layer.
