You need to have the following properties set in your Notion Database:
Date (format date)
Notion Automations Events Data (format text)
You will need to share that database with the Notion Automations Integration, by clicking on "Share" and then selecting "Notion Automations".
On your Notion Automations Settings, Go the "Notion Connection" Section, and refresh the calendar databases to pull the newly shared database. And then select your custom database.
Now the synchronization will be linked with your custom database.