What it does
cds add approuter puts the router in the project, and cds bind --exec -- npm start starts it on port 5000 with the credentials of the bound XSUAA instance, with the service running beside it under the hybrid profile.
Conditions
an XSUAA or SAP Identity service instance exists in the cloud and is bound with cds bind, since the credentials the router starts with come from that binding; the CAP application runs beside the router with the hybrid profile
Remaining work
Create the service instance and the service key in the cloud, bind them, and keep two processes running. The router itself needs no restart on a file change, since it serves static files or delegates to the backend service.