How to use default vault with aws_backup_plan

The rule for an aws_backup_plan requires specifying a target_vault_name, and for a cross-region backup, I need to specify copy_action.destination_vault_arn.

However, I haven’t been able to find any documentation, either for AWS for the the AWS terraform provider for what I am supposed to use here if I want to use the default vault. How can I do that?