在AWS Backup中指定要备份的卷需要使用AWS Backup API。 下面是一个Python示例代码，用于指定要备份的卷：

import boto3

# Create client
client = boto3.client('backup')

# Specify which volumes to backup
resource_arns = ['arn:aws:ec2:::volume/']
backup_plan = {'Rules': [{'RuleName': 'BackupRule', 'TargetBackupVaultName': '', 'ScheduleExpression': 'cron(0 12 * * ? *)', 'StartWindowMinutes': 60, 'CompletionWindowMinutes': 10080, 'Lifecycle': {'DeleteAfterDays': 30}, 'CopyActions': [{'Lifecycle': {'DeleteAfterDays': 365}, 'DestinationBackupVaultArn': ''}]}]}

# Create backup plan    
response = client.create_backup_plan(BackupPlan=backup_plan, ResourceArns=resource_arns, BackupPlanName='', BackupPlanRuleName='')

以上代码使用boto3库中的AWS Backup client创建备份计划，并在ResourceArns中指定要备份的卷的Amazon资源名称（ARN）。backup_plan参数指定备份计划的详细信息，包括备份窗口时间，删除后的生命周期和复制操作。可以根据需要修改这些参数。最后，create_backup_plan（）方法用于创建备份计划。