Dynamics 365 Server, version 9.0 is available for download now.
It’s possible to connect Dynamics 365 version 9.0 on-premise with Microsoft Flow:
- Triggers: make use of webhook registration to post a request to Flow connector ‘When a HTTP request is received’
Make sure that in both cases the calls are not blocked by the firewall. Here is the info about IP addresses and domains that should not be blocked on your network: IP address configuration
BTW: there is a Dynamics 365 connector for on-premise planned.