You could use our "REST API - POST Create Task (Backup)" to trigger the backup with your specified retention type.
||The option to set job retention.
Valid values are:
- INFINITE, to retain this job indefinitely
- NO_OF_DAYS, to prune the job after the number of days specified in the numberofDays parameter
- STORAGE_POLICY_DEFAULT, to apply the retention rules of the associated storage policy
Above mentioned is our official documentation link.
Alternate link: https://apidocs.commvault.com/#942f81d4-f7b9-95ea-9e41-c23a55348056
For more info: http://documentation.commvault.com/commvault/v11/article?p=features/rest_api/test_environment_postman.htm