I am not an expert but I guess the event data should be ingested into AEP via a streaming data source. And for that you should leverage Adobe Launch and configure web sdk (if source is web) or mobile sdk (if source is mobile app). For example- If your journey starts when some user adds something to a shopping cart. Then that cart add event should be sent to AEP via Adobe Launch web sdk. If it is not a behavioral data and coming from an external third party source then you will have to ingest that data by configuring your data source manually.