Hello Gagan, have you read this
https://www.eventbrite.com/developer/v3/api_overview/authentication/ ?
1) Create an app and define the URI to get that callback to your domain
2) Redirect the user
3) The user needs to login an explicitly give your app access
4) The URI defined will be called with the user token
5) You could start doing requests to the eventbrite api using the user token, like the user details for example, or the list of events, etc
Thanks