The definition of a bounce is subject for another discussion but for this example you define a Bounced events filter where Session Start equals 1 and Session Ends equals 1. You use this filter do define the rate of bounced sessions to sessions.

Filter

Calculated metric

Derived fields

Alternatively, you can define a bounce rate using derived fields.

Derived fields are part of a Data view which has the advantage that not every user can override or modify the definition of a Bounce rate metric. That advantage also introduced a limitation. Users that do not have access to a data view cannot use derived fields and have to resort to filters and calculated metrics to define a bounce rate.

See for more background information on how to calculate bounces and bounce rate in Customer Journey Analytics, this blog post.