post https://<<tenantUriId>>.api.chassi.com/<<applicationSpaceUriId>>/lifecycle/<<serviceUriVersion>>/journeys
This, quite simply, will start to track a journey in the designated Operational Application Space. Be careful to make sure the journey is tracking the appropriate entity, otherwise you will end up with journeys which have no site access. This may be intended in some cases, but usually a customer, either internal or external or both should be connected to the journey.