CDK how to use reserved rds/ec2 instances?

i purchased a EC2 and a RDS reserved instance. how do i reference it in CDK. and if i wanna use the reserved instance for auto scaling group. is it possible? how do i reference to the reserved instance id in auto scaling group?

const asg = new autoscaling.AutoScalingGroup(this,`${props.project}-${props.environment}-ASG`,{
      vpc:this.vpc,
      instanceType:ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE2, props.environment == 'production'?ec2.InstanceSize.SMALL: ec2.InstanceSize.MICRO),
      machineImage: new ec2.AmazonLinuxImage({
        generation: ec2.AmazonLinuxGeneration.AMAZON_LINUX_2
      }),
      securityGroup:securityGroup,
      userData:ec2.UserData.custom(userData),
      minCapacity:1,
      maxCapacity:1,
      desiredCapacity:1,
      role:role
    })

i tried to search in CDK documentation and google. i could not find any answer. all i found was the following link

You do not have to do anything. If the instances in your ASG match the specifications of the Reserved Instances that you bought, it will be applied automatically. From docs:

If you purchase a Reserved Instance and you already have a running On-Demand Instance that matches the specifications of the Reserved Instance, the billing discount is applied immediately and automatically.