Under Function App Networking features, you can also configure hybrid connections. Hybrid connections allow the Function App deployed into App Service to connect to an application endpoint on another network. Each hybrid connection is configured with a single TCP host and port. Hybrid connections allow only for connections from your App Service to an on-premise (or other) network applications, not vice versa. These connections are configured using Service Bus Relays.
We will not elaborate on the hybrid connections setup. To learn more, please visit https://docs.microsoft.com/en-us/azure/app-service/app-service-hybrid-connections.