get_auto_scaling_group.ts

// Create your ECS cluster, don't use the capacity option.
// We will add it in a different statement so we can keep the returned AutoScalingGroup
const ecsCluster = new ecs.Cluster(this, 'ecs-cluster', {
    vpc: myVPC,
    clusterName: 'my-cluster',
    // ...other props
});

const autoScalingGroup: AutoScalingGroup = ecsCluster.addCapacity('cluster-capacity', {
    instanceType: InstanceType.of(InstanceClass.T2, InstanceSize.LARGE),
    keyName: 'some-key..',
    desiredCapacity: 2,
    // ...other props
});