Once you've listed your event on JustGiving, you’ll receive an email to confirm that the event is added with your event ID provided. You can also look up your event ID in your charity account by following below steps:
Log in to your charity account (URL: https://www.justgiving.com/sso)
Click on the Events & Widgets tab and select View & promote events
You’ll see a list of events that have been added to your profile page. Look for your particular event and you’ll find the ID just under the event name: