At runtime you must use the blueprint methods listed in the documentation linked to access the datasmith runtime functionalities. There is no UI linked to it so you have to make your own widgets if you want to do some source browsing and management.
As a first test you could just do a function that browse the sources and open the first one.
You can get an example if you look at the UI in the Collab Viewer project: in Content/CollaborativeViewer/Blueprints/Commands/DataSmith/UMG/ starting with the DREndpointUI widget.
Custom runtime application has some network communication limitation and you will get no sources listed by the blueprint function. This is where the “-messaging” option comes in to make sure all network functionalities are enabled for your runtime application.