If you're passing in a payload via the request body for syncing to a target connection, your process flow will need a map shape - source data fields (for your incoming payload) should be added manually.
The initialise flow API request requires two identifiers - one for for the flow to be run and another for the specific flow version.
To find the process flow id, access the required process flow from the dashboard. You'll find the process flow id above the title at the top of the canvas:
And you'll find all process flow version ids at the bottom of process flow settings: