Which service for autoscaling automation

Hello.

Why would you want to increase capacity only in a specific AZ?
Can't you solve this by specifying only a specific subnet from the beginning without using Lambda etc.?
https://docs.aws.amazon.com/autoscaling/ec2/userguide/create-asg-launch-template.html

+1 to Riku's questions, what's the usecase here? There might be better solutions for your underlying problem

But to answer your question, it depends on exactly what you're doing. You can't set min/max/desired on the ASG per AZ. If you're trying to shift all traffic from one AZ to another, then you could use a Lambda to change the subnets in the ASG, so that you remove the current subnet in AZ1, and add a replacement subnet in AZ2. The ASG will the automatically rebalance the instance into the new AZ