Workflows are triggered by an Event.
To link a Workflow to an Event open the Workflow and click on the Start Action.
If an Event has already been received by the API/SDK or manually added via Data > App Events then it should show up in this blind. If it doesn't you will need to go and create it first.
Check the box next to any Events you want to trigger the Workflow.
Events can be linked to 1 or more workflows.
Multiple Events can be linked to a single workflow.
Anytime this Event is hit from now on, the associated userid will be passed into the Workflow and will follow the Journey.